The cost is down into hobbyist range. Not the cheapest hobby, but not a big deal for a lot of people. Look up USRP, HackRF, and BladeRf. I think BladeRf is the cheapest at $420.00 USD.
Putting up a single, limited range BTS is not the same as full scale, functional GSM network. It's similar as creating your home network with OpenWRT flashed router and calling it Internet.