For Pentax there are a lot of cheap primes, but most are second hand. The new DA-L 35mm F2.4 is a good cheap prime (almost like 50mm on FF). The limited primes are great, but more expensive (but worth it). I own an old manual 50mm F1.7. It is high quality! There are also autofocus ones F 50mm F1.7 and FA 50mm F1.7. Those have the same price as the Nikon and Canon ones. There are also very good second hand 28mm F2.8 primes. If you like old analogue primes for less than $100 Pentax is great. Many of those lenses are really surprising.
I currently own the DA* 300 and M50 F1.7.. my next lenses will be the DA 35 F2.8 limited or FA 43 F1.9 Limited. Both are excellent. I will also buy the A28mm F2.8 with some macro rings and an a turning ring. It gives me a 3:1 macro for around $120.