MOSIS at USC has a service where they combine designs from many groups onto one wafer and get it fabricated in a variety of processes. We have had some pretty complicated designs done through them. A medium-size chip on the ON Semi C5 process runs us about $25000
for 40 chips. Adding more chips costs a couple thousand $ for 40 more. Once you have the design 100% perfected, you can have them do full wafers of just your design at a much better price.
One trick is to just order 40, and then get all the rest of the parts off the wafer and possibly additional (safety) wafers that were run. These chips will be available very cheaply, but cannot be counted on to have made it through the process.
Watch out, the packaging fees can also be expensive.
Jon