Car losing oil

I have a 2001 chevy cavalier with a 2.2L engine that has a little over 150k. A while ago it developed a small oil leak, leaking about 1/2 a quart every other week or so. That did not concern me much so I did not worry about it. However in the past few weeks it has begun to lose oil at a disconcerning pace. I have not noticed any more oil leaking on the ground where parked then when the leak was smaller. However, the vehicle is losing about as much, if not more, every day then it did every other week. In fact one week I filled the oil on monday and by thursday I had to put almost 3 qts in it. What could be causing such a leak? What would be the cost of such? Is there anything fairly simple that I can do to eliminate that a cause before more expensive measures?