HackRF is the gold standard, but definitely not cheap. I have an SDR-Play duo that I'm pretty happy with, writing software for it is doable (even though the manufacturer is really coy about what's in there you can dig into the driver easily enough). It remains to be seen how long those will be for sale though, I'm not sure if they're still making the chips (long story short: the SDR-Play seller once upon a time made a chip for another market and they seem to have realized they could make more money selling those same chips packaged to the SDR market).