Backpacking bags are usually pretty expensive. Shaving off the ounces can shoot the price up in a hurry. That said, if you're not counting every last ounce and want to save a considerable amount of money on your pack, take a gander at the Teton Sports Scout 3400. It's an internal frame 55 liter pack made of 600D diamond ripstop with a 600D Oxford canvas shell, and it has a built-in rain fly.
Currently, the Scout 3400 is about $15 off its usual price at $55.24 for the Hunter Green version. Both the Coyote Tan and Mecca Orange versions are $59.99 -- still about $10 off the typical price. At 4.5 pounds, this pack won't be the lightest on the trail, but you can use those savings to splurge in other areas, like a quality down blanket or lightweight tent (the Kelty Circuit 1P Tent is still 50% off at REI).