I will say this, though, if you're making simple meat and potoates stuff, it certainly is a lot cheaper that eating out. I could feed a family of six on chicken paprikash with homemade spaetzle, and I don't think it would cost much more than $10, given the insanely cheap price of chicken at the local market.
It's when you start buying items like avocados ($1.50 a piece at the Mexican market by my house), dried chiles ($2.50 for 2 oz), and such semi-specialty items that the costs start adding up quickly. If you like to cook ethnic cuisines, a lot of times it requires going to ethnic supermarkets to get reasonable prices and save money over going to a restaurant. For example, veal is expensive at Jewel and Dominick's, but bloody cheap at most Polish places (something like $2.50/lb last I checked.) If you have a taste for lamb, it's insane at the supermarkets, but go to a Middle Eastern or Pakistani neighborhood, and you could find it for a fourth of the price. Once again, at around $2-$3 a pound, rather than the $10/lb I see around.
And if you want to cook all organic, using produce and ingredients only from Whole Foods or farmer's markets, then it's not difficult at all to imagine eating out being a cheaper option for some people. Granted, for the quality of food I like, I'll do better cooking in than eating out for the same price, but I could get a decent enough prepared meal for the cost of ingredients if I shopped at places like this all the time.