For dinners, assuming I'm actually cooking (read: throwing crap into a crock pot), I can do it for around $10, and that usually will also leave me with enough food left over for 2 or 3 more meals. For example, tonight I'm making a chili and the main ingredients (beans, meat, tomatos, etc) cost me $13.18- though looking in the pot, I'm pretty sure I'm going to be eating chili for the next 3 weeks! What's killing me, as someone who's just starting out on my own, is buying the various "one-time cost" things, such as the spices and the sauces and the things that you don't need to buy for every single meal. That's driven my costs up a few times.
My normal dinners are pretty cheap ($0.88 TV dinners or $1 Pasta-roni boxes), and then I'll try to do the real cooking once a week.