Gone are the days of the five dollar buffet in Las Vegas. At the strip casinos it's pretty much impossible to find any sort of lunch entree for under $10, and it's even more than that for dinner. Many of the casinos are trying to upscale everything and the proliferation of $12 hamburgers is both impressive and amusing, since exactly the same thing at a regular restaurant will cost you $5 less.
Fortunately there are a couple of pockets of cheap eats on the strip. Casino Royale has a Denny's and Subway inside it, and the little food court between there and Harrah's contains a McDonald's, a Chipotle and a little pizza place with prices like those back home. There's also a McDonald's between the MGM and Planet Hollywood in one of the less-developed parts of the strip. The Fashion Show Mall (yes, that's the real name) across from the Wynn has a food court with various cheap eats as well.
If you enjoy spending $3.75 for a bottle of Mountain Dew, by all means buy one from a kiosk in the Paris. However if you would rather spend your money on video poker and strippers, visit the Walgreen's or CVS between Planet Hollywood and the MGM (they're on opposite sides of the street, so you're set either way) or head down to the ABC Store in the Fashion Show Mall. All three have reasonable souvenirs as well, although the shirts at the ABC store seemed nicer to me.
I'm sure there are other low-priced options that I forgot about. If you know of others, post a comment so others can have a good time without spending an arm and a leg!