Best I can recommend in the exact $50 range is:
1) Corsair SPEC-01 RED (with window)
2) NZXT Source 210 (no window)
For $65 you could get the Corsair Carbide Series 200R (no window, but good case).
For $75 you could get the NZXT S340 w/ window...also a very good case.
All above cases are atx mid-tower.