If you want good cooling performance, a 120mm intake fan and a 120mm exhaust fan is the absolute minimum for fair performance. If you want good cooling, look for two 120mm intake fans, and two exhaust fans. Usually, the second exhaust fan will be on the top of the case.
Your budget of $50 is marginal, unless you can find a case which includes:
1) free shipping. Cases are heavy, and shipping can be $20.
2) Are some fans are included? If not, expect to pay $10 to $20 for each extra fan of decent quality.
3) Is there a rebate or promo code available?