Due to a marketing job, I know more about home goods than I care to! But
in my experience, better quality sheets (but not super fancy ones) last many times longer than cheap ones. I'll never buy cheap ones again.
Percale (plain weave) wears longer than sateen.
Thread count means little. These days they can spin really fine thread from absolute crap cotton, meaning sheets that seem fancy initially, but wear thin and tear in no time.
I'll go along with The Sweet Home's recommendations:
http://thesweethome.com/leaderboard/bedroom/The LL Bean ones are awesome, and super durable. Another favorite of mine are the Lands End ones made of oxford cloth. They take a few washes to soften up, but after that they're great.
Marshalls is good too. Look for name brands like Hilfiger, Ralph Lauren Polo or Laura Ashley. At Marshalls they're the real thing discounted, vs. made-for-the-outlet crap. You can put together nice queen sets for $60-70, especially if you don't mind mixing colors.