Comments/Ratings for a Single Item

Well, at least as far as we need to come together to justify here is a market for such pieces.  On this front, I would say an initial round of pieces to be done would consist of:
1. Chancellor
2. Archbishop
3. Amazon
4. Cannon
5. Vizer

I would add Elephants to that list. I don't mind the Xiangqi Horse being represented by a normal Knight (although for experienced Chess players the difference between the two is the biggest stmbling block for playing Xiangqi). But I am not sho charmed by the representation of an Elephant by an ordinary Bishop.

As I pointed out elsewhere, most of the pieces you mention are quite easy to make from a normal Staunton Chess set. I am not sure what you mean by 'Vizers', but I assume you mean the pieces next to the Xiangqi royal piece. They could be decapitated King and Queen of a normal set. (I don't think the 'head' they have here in the photograph, somewhat reminiscent of an old-fashioned coal can, adds much esthetical value.) The Queen's head could then be glued on top of a Knight, (after grinding off the ears), to make an Amazon. The upper part of the other Knight could be glued on top of an inverted Rook to make a Chancellor. A V-shaped cut in the Bishop's head (making use of one of the cut that is aready there)could remove the top of the Bishop, and make it an ArchBishop.

When done carefully, all this would provide high-quality pieces that blend in perfectly with the normal Chess set they were made from, and starting from two equal sets would automatically supplement the number Pawns you ned for wider boards. You would actually have a surplus of Pawns, and a Pawn glued on top of a Knight's base could be another general-purpose piece. (Unfortunately you can't make a pair of those, if you use the other Knight for building an Amazon.)

Biggest problem are Cannons and Elephants. The Cannon's could be made from Knight's bases plus some parts specally manufactured for the purpose. To make something that remotely looks like an Elephant, even in an abstract sense, is a problem in itself.

If we would really want to sell such pieces through this website, the best way might be to strike a deal with a manufacturer of normal Chess sets, to order pieces or piece parts individually, so that we are not limited to the frequencies (or sizes!) with which they occur in normal Chess sets. We could then, for example, use a Bishop of a larger size to convert to Archbishop, and we could buy the Knight heads and bases separately. Unlike plastic pieces, wooden pieces are manufactured completely independently, and not having to assemble the Knights only saves the manufacturer money.

I have looked at retail prices of wooden Chess sets, and some acceptable sets are already sold on the internet for €16,90. (I could even find sets for €7,99 in the toy shop, but they are of ugly design and rather smal size.) This amounts to ~50 cents per piece. In general, a manufacturer should be happy to sell his products for the retail price.

The following manufacturer in China creates molds for customized plastic pieces (look under 'customized chess') :
http://www.cnchess.com
/Mats

I hate plastic pieces anyway. If you want custom-made pieces, I still think
wood is the way to go. Like the Superchess pieces, which cost $3-5 per piece,
but can be manufactured in quantities of 10-100.

I agree with dhr. Muller in my preference for wooden pieces.  Nice ones are not very economical, and custom designs would be quite expensive.
I did a 'back-of-the-envelope' calculation and came up with a price of about 2-4 USD per piece if executed by an experienced production hand turner, cheaper if the design and quantity allowed the use of an automatic duplicator.  I checked this against prices in internet stores for hand-turned lace bobbins, which are similar to chess pieces in size and complexity, and undecorated ones are about 3-4 USD.
There are other less traditional ways of making wooden chesspieces, notably ring-turning and scroll-sawed sillhouettes, which might be cheaper.
For those who might want to butcher manufactured sets, the cheapest chess piece retailer I have found online is www.wholesalechess.com

As to the economy of the operation: From a single normal Chess set ($30 if you buy them
in 4+ quantities), you can manufacture two Capablanca supplements (Chancellor, Archbishop,
and 2 Pawns), after which you are left with 2 Knight bases, 2 Bishop bodies, 4 Pawns, and
K and Q heads. The latter can be combined with the Bishop bodies, to make 'mini' Queens,
which can be used as Advisers in Xiangqi, and mini Kings, which can be Commoners for Knightmate.
Making 4 Capablanca supplements produces 2 Advisors, and a separate Chess set is needed to
provide the Rooks for making Cannons (using 2 of the 8 superfluous Pawns as bases).
Two other Pawns go on top of 2 of the 4 Knight bases left over from the Cancellor production.
These 'giant' Pawns can be used as Xiangqi Elephants.

The 4 Capabanca supplements also leave 2 mini Kings, and one of the Knights of the
third set can be put on top of another Knight base to make a Royal Knight. This completes
a Knightmate set.

An alternative would be to scrap the mini Queens, and play Xiangqi with normal Queens for
the Advisers. The set that provided the Cannon material can also provide an unmolested Queen.
The two Queen heads left over from the Capablanca sets could then be used to turn the
Knights of the third set into Amazons.

So from 3 Chess sets ($90), you woud make:
4 Capabanca supplements (Chancellor + Archbishop + 2 Panws) ($16?)
1 Xiangqi supplement (Queen + 2 Cannons + 2 Elephants) ($20)
1 Knightmate supplement (King + Royal Knight) ($10)
1 Amazon ($3)
2 mini Kings ($5 the pair)
2 spare Bishops
12 spare Pawns

Of course it would be possible as well to play with Bishops for Advisors Then you would not even need the extra Queen.

There are some three dimensional Chinese Chess Sets available:
Amazon - $37.40 - three left! - Romance of the Three Kingdoms
http://www.amazon.com/exec/obidos/ASIN/B000Y92XX4/ref=nosim/tbook-20
Amazon - 65.90 - Monkey King Set
http://www.amazon.com/Monkey-King-Chinese-Chess-Xiangqi/dp/B000SOJ2R0
/ref=sr_1_70?ie=UTF8&m=A3IXHP9WB8UQLD&s=generic&qid=1232511651&sr=1-70

Here is a three dimensional Chinese Chess Set similar to a Western Chess
Staunton Set - Don't know how much it sells for
http://www.china-suppliers.org/prod_img/a1/1001283/vender_e_100006968.html

Here are two more three dimensional Chinese Chess Sets similar to a Western
Chess Staunton Set - Small -  Price: £59.95 (£52.13 ex VAT )
                     Large -  Price: £89.95 (£78.22 ex VAT )
http://www.paydaygames.com/acatalog/Xiangqi_Sets.html

Yet another three dimensional Chinese Chess Sets similar to a Western Chess
Staunton Set -   Price: £65 - Large set: 35.5 x 30.5 x 2cm, King size
4.2cm
                   
http://www.zonzab.co.uk/acatalog/Chinese_Chess_Xiangqi.html

Play Chinese Chess on your Palm Pilot with Chinese Chess Sets similar to a
Western Chess Staunton Set (or in Chinese writing) -   Price: shareware
Register for $12 if you like the game.
                   
http://palm.pair.com/palmcc.html

Here is a fine example of a wood set that includes a canon (styled as a
canon) as the rook. Don't know exactly what other sets would match well
enough to have a start on a Chinese Chess Set. 
www.chessbaron.biz/chess-M2011.htm