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There some projects to realize low-cost, short turn-around-time LSI fabrication at slightly old process, such as 1um, in Japan, whose name is Minimal Fab. This will enable the (commercial) LSI fabrication service at a few hundred USD cost and 1-week TAT. I believe this a revolution similar to that happened in PCB world in ten years ago.

Wikipedia says prototype runs are done MPW or MLM, with many different chips printed on a single wafer. Nobody quotes actual prices, of course, so guess "a lot"

https://en.wikipedia.org/wiki/Multi-project_wafer_service

These do multi-project wafers (aka shuttle runs):

https://www.mosis.com/

http://www.europractice-ic.com/

FPGA conversion has a 50k nre. Anything else and you need a team of people on your end to do things like verify quality.

The fun part is that you can sometimes sneak an trial design into a shuttle service. ( but good luck making that initial design!)