hard to say, If u want a cheap wheelin vehicle, a YJ is a good choice. Its tuff to tell you the cheapest, only because there are so many factors involved in the Jeep, especially the YJ. the 2.5 and the 4.0 motors run forever, so dont focus so much on the odometer, check for rust on the floorboards, and on the chassis (around the steering box and on the bottom side of of the chassis above the rear wheels).
I think, you could find a Jeep for $1000, that has under 190k miles, with minor rust. But i would be a little hesistant keeping it as my daily driver. I paid 3grand for mine. It had no rust, 185k miles, and a 4cyl.
to answer your question, im going to say 1500....and if you are serious about buying a jeep a few more hints..test drive it, look for oil leaks, look at the chassis (where i said) for rust, and DO NOT BUY A JEEP THATS ALREADY LIFTED!!