MOSIS (https://www.mosis.com/) will do runs of 20-100 ASICs. I've never used them but I just requested a couple of quotes, and it looks like their prices range from a few thousand dollars to a few tens of thousands, depending on process and size.
In the MEMS world we have something similar to this where you don't get a full wafer you just pay for copies of the device you design to be put on the next run. In this case it makes even more sense because the processes are much more standardized for IC work.
