And you should be able to get much more for a kidney, assuming you haven't been abusing it.
Re: rigid mountain bikes, they are now something of a niche and so they're not any less expensive than any other "good" bike. An example is the non-suspension version of the Surly Karate Monkey, the ones currently available blue and black:
https://surlybikes.com/bikes/karate_monkey
And they're often also single speed, for maximum grumpiness:
https://surlybikes.com/bikes/lowside