Comment by namibj

OsmocomBB and LimeSDR would like a word with you. Yes, the former is limited to GSM, the latter doesn't come with a TX amp and you'll need to supply suitable mid-power RF (no cooling for passives, carefull cooling of actives) antenna circulator/filter/switch, if you want to use your new amp. The hardware should be under 2k$ manufacturing in single-unit quantities, but it is HF design, including some distributed-element filters and power-handling at low GHz frequencies. Nothing particularly trivial to design, though the requirements in precision are not too stringent, so you won't need someone who can demand >100$/h while working outside of a major metropolitan area.

TLDR: GSM+LTE open-source SDR/hacked dumbphone baseband exists, suitable hardware is COTS for sub $2k.