Food costs compared to Gasoline riding cross country USA
I rode the Southern Tier last year and used a GPS app on my phone that estimated caloric output. Probably not very accurately, but it gives us a number to kick around. My distance was right around 3000 miles.

Total estimated calories burned over the ride was 143,000. That would be equivalent to 265 Big Macs (540 cals per Big Mac). Not that I ate any Big Macs, but it's a handy yardstick. A Big Mac costs about $4. So that would be $1056 in "fuel" if that's all you ate. I'm assuming that you would burn about the same number of calories whether you rode a relatively small number of high-mileage days or a large number of low-mileage days (I think that technically this is wrong because in the latter case you've got more off-bike time during which you're also burning calories).

Interestingly, a gallon of gas costs about $4 right now too, depending on where you are. If you're in a car and getting 22 mpg over the same distance, and filling up with $4/gal gas, you'll spend $545.

Although I did keep track of my food costs at the time, I no longer have that data handy. I was not cooking a lot of my own food—mostly eating out. So my food costs were higher than they had to be. On the other hand, you could not get away with just pasta--you'd need a lot more protein and fat to keep going over the long haul. As it is, I lost about 15 lb.