I have never used this specific material, but the Smooth-On folks are pretty cool to work with.
http://www.smooth-on.com/index.php?cPath=1156
I have no experience with this specific manufacturer:
http://www.dickblick.com/products/castincraft-clear-polyester-casting-resin/
Having dabbled in the mold making - your upfront investment will be about 100 - 200 bucks for materials and supplies. Add in the time to learn how to make the molds work. Divide that amount by the number of pieces you want to make.
There's your ball park cost.
If you want more detailed info, PM me - I'll be glad to share what I have learned.