If you can mail order, Nashbar, REI, Bike Tires Direct and a few others have frequent discounts on good tires. Lots of choices under $30. Some will offer free shipping for minimum order amounts, and occasionally they'll even waive the minimum order limit -- usually it's $49 minimum with Nashbar, but occasionally they'll waive that for some special 24-hour sales.

I like Michelin Protek Cross Max, if you can still find 'em. Or the Protek Cross (not Max), with the thinner 1mm puncture shield should be good as well. Tough, grippy tires. I've ridden mine for about 10 months on mixed terrain, rough pavement, gravel, even across unavoidable patches of broken glass. No problems. They're heavy but tough. No idea about long term, whether they'll be good for years without sidewall crackling from age. But they've been tough over the past year.

Michelins are good values because they don't get word-of-mouth recommendations as often in the US as Schwalbe, Kenda and others. So they seem to be discounted more often.

Otherwise, ask the local bike shops if they have any extra used tires in back for sale. Some serious cyclists with money will have the LBS put on new tires often and don't care what happens to the old tires.