I have found that my Canon 28mm F2.8 performs pretty well on crop sensors and is cheap + cheerful. I do prefer my 25mm F2.8 Contax-Zeiss (with a 10 GBP adapter) is better but it's entirely manual - does not suit everybody.
Unless I had bad copy, most photos I took with 28mm were soft-soft-and more soft.
@ OP, Canon Ef-S 17-55 IS 2.8 is one of the best lens for crop. It's little over your budget. I hope you can find a decent used lens on CL.
I have the 18-50 2.8 and find it's dof isn't narrow enough for what I'm looking to get, hence why I'm looking at primes.
If you want to get the narrow DoF, try getting closer to your subject with it at 50mm @f/2.8. The DoF tends to get narrower as the subject gets closer to the camera. That's about the only way you can get stuff thrown really out of focus with a super-wide like 14mm or 8mm fisheye, get the subject really, really, really close to the lens, then the background manages to get far enough out of the acceptable DoF you can actually see it.