'I think they are too expensive. How do I make my own ball joint doll?'
Firstly, it's not as easy as you might think to sculpt and cast your own doll, and it's definitely not cheaper. My suggestion would be to go back to the first chapter and re-read the section on how BJDs are made before deciding that you want to make your own. Secondly, casting can be dangerous since resin is partially composed of Isocyanate, which by it's name is probably related to Cyanide and is therefore poisonous. It can also be expensive once you take into account the tools and waste, especially if you have no experience in casting; not to mention the cost of the supplies themselves. You also need a clean area that is free of moisture and humidity and has a fairly steady temperature in which to work, (72-76 degrees is best.)
Hopefully you have some experience with both sculpting and casting but if not, the best way to learn is by doing. Get some Sculpey or LaDoll and play with it for a while before setting down to do your actual doll parts. It would be even better if you could try small amounts of several different kinds of clay before settling on the one you'd like to use. There are a lot of options and you aren't obligated to use any single one. Some people even carve their doll prototypes in wood, polystyrene or other materials instead of sculpting. I have tried to include some of these tutorials too. Test casting some smaller resin parts first is also a good idea before you go full scale and helps to work the kinks out of any system you use so you don't have as much waste; it also makes the entire process less costly.
Last of all, sculpting and casting your own doll can be very rewarding. If you can make it past some of the more technical problems that can arise and the (nearly prohibitive) cost and end up with a complete doll of your own creation, then major kudos to you. It's definitely not easy, but it is quite the accomplishment.
If you want to learn from others, join the forum: The Joint.
Note: many of the tutorials presented here are not BJD specific but instead deal with how to cast polyurethane resin or silicone molds. The technical details still apply.
Classes, Books & Videos 4 Sale
June Goodnow Moldmaking & Resin Casting Video
Dollmaking Classes by Lumicast
BJD making book
'Yoshida Style' by Ryo Yoshida
Dolly Daydream: Coppelia
Photos, Instructions & a Shopping List
Making Noah's Doll
A complete run through of the process
A Translation of how Shuen made Safi
A Translation of a LinkClub Tutorial
A Resin Casting How-To
Bishonen House Mold Making Tutorial
Donn shows you how he makes his molds.
The tutorial Fancyboots used to sculpt Liebchen
Resin Casting Videos on YouTube
A preset search for videos on resin casting on YouTube
How to Sculpt a Head & Make a Mold
Not in English, but has photos
*Translated* Haru's Doll
An extensive site dedicated to making a doll
Japanese(?) photo tutorial with translation & many tips
A very complete guide to casting in resin
A beautiful PDF about casting in Resin
Victorian Model Railroad Society
A Tutorial on casting in resin at home
Search for suppliers in your area!
Pour A Kast & Pour A Mold
Molding/Sculpting/Casting Vids & Supplies
Complete Site with How-to's, FAQs, Supplies, Tools and Videos
Bare Metal Foil
A dealer for several casting resins
I also recommend various Taxidermy or Modelers Forums as they often contain a lot of useful info and people you can ask questions about molding and casting.
Links that may also help:
"How Can I Make One?"
Resin Casting Links
Tutorial on How To Make a Vacuum Chamber
Uses PVC Pipe
Another Tutorial on How To Make a Vacuum Chamber
Uses Pyrex Bowls
She will cast your dolls for a fee