Purchase prices fluctuate quite a bit based on condition and other things, but I think a pretty fair median price for a 150 is about $25000. For an old 172 (1985 and earlier), I would say that $50000 is probably pretty fair. Of course a new 172 (post 1997 when they started making them again) will be much much more. Insurance costs will be more for the four seater, fuel burn will be about 8-9 gph instead of 5-6, and maintenance will probably be only slightly more.