I bought a i7-3770k from ebay for $50. However, trying to get a decent motherboard was a nightmare, took 3 months of constant searching and finally got an msi mpower z77 for $100.
Honestly, best bang for the buck for a total pc is shopping Craigslist or ebay for the full thing. Trying to part out a full pc now, especially with older gen Intel is far more expensive. You can get functional pc's in the want-adds for $150, if all that's needed is a windows, surfing, YouTube machine
Amazon has full functional, brand new pc's with OS for $200