I third the recommendation of gap, after sonick and Jon. I've got seven or eight of their athletic fit basic tees in different colors and find them great for the price (the regular 100% cotton ones are good, as are the ones with a little stretch). This coming from someone who always used to buy Helmut lang's egyptian cotton tees, thinking that I couldn't ever get the fit anywhere else. Geesh, THAT was stupid.
Gap shirts are SLIGHTLY less in quality than the Lang and the cotton is just SLIGHTLY less soft, but for $10 as opposed to $110, I quickly learned to go to Gap. And, the fit/cut is similar. One of my off-white basic Gap tees, set next to my Lang off-white basic tee, if almost indistinguishable (except for that huge Prada Group tag in 50 languages sticking out the bottom side).