All depends on your price range, mechanical ability, comfort level ect. The numbers I am quoting are real in my area, there is always someone telling you they can get a new JD 4320 with loader and mower for less but lets be realistic here, you may get it for a little less, but not much. As for used, I can find a pile of junk, or buy one from my uncle for a little less, but realisticly for a working tractor on the street?
New JD 4320 is what I want, but will probabaly never get.
$30,000
JD 790 (3005 now I think) is what I got, real nice 4x4 with loader, works hard, easy operation and maintenance, reasonable price.
$20,000
Thousand series Ford, New generation JD 1020, real nice older tractors. Independent PTO nice option, not as comfortable, no 4x4, heavier for mowing lawns, real workhorses.
$7000
Ford 8n, Fergusen TO, still real workhorses, transmision type PTO, I work withan NAA (next gen 8n) weekly, even choose it over my JD a lot of the time.
$2500
Farmall H, Allis Chalmers WD, same as 8n except no 3 point hitch, Implements must fit tractor and are getting harder to find. I also have one of these and use it a lot for drawbar work (mowing hay, baling hay, chopping green forage, ect) but it is much more limited. You can get aftermarket 3 point hitches for them for $1000 but thats a lot of money to put into an old tractor.
$2000
Just my 2 cents. Good Luck!!!