The two organization for short run prototyping IC service I know of are:
www.mosis.com -> US one
www.europractice-ic.com -> EU one
My impression is that both services are directed toward academia but accepts commercial/independent projects too (I know that people at my Alma mater used both services with success).
For basic info about prices check out the link:
Both provide wide range of technologies - from quite old ones to quite new ones. I am leaving the work on learning for what can be done at what technology for you. However basic info is that in .18u tech one can pack about 100k gates per sq mm. Going up or down with technology node will roughly double or half of the gates per sq mm. So at .13u you will pack 200k and at .25u you will pack about 50k gates per sq mm.
This is for prototyping (that is you will get ~40 dies for each run), for production you will need to talk with foundry or specialized provider directly. Expect all hurdles attached with dealing with big business (NDAs, lawyers, will talk with you only if you want to spend big $ with them etc.)