Only you can answer that.
Some cheap foods:
ground beef - fattier is usually cheaper AND more energy dense, a win/win
big jugs of olive oil
Buy in bulk and take advantage of your freezer, just get some freezer paper and freezer bags, freezer burn sucks.
Look for specials in the Sunday paper: I recently found whole grass-fed ribeyes for $4.99/lb - they are usually twice that.
If you figure it out, report back.