Thanks for your reply, Eric. I'm interested in hearing what owners are experiencing as far as ongoing costs are concerned.
Insurance, tiedown, fuel, oil, etc... are easy to predict (I have them already). I plan to keep the plane for 300 hours give or take, then sell it, hopefully recovering the majority of what I pay for it. I can afford routine service, but it's really going to hurt if I have to replace the engine or have it overhauled.
My last annual was about $2,500, but I had them do some extra stuff (that he said it would need next year). One thing I learned for sure, look at the airframe as carefully as the engine. While shopping for a 150 or 152, I was mainly concerned with the engine time. In the end, I actually bought the plane with about 1600 SMOH and 550 STOH. Note that many O-200s will need a top end around 1100 hours and you will see them for sale like that. The mechanic is convinced that this one will go well past TBO safely. There are never any guarantees and I've heard of engines needing overhauls well before then.
The last owners sank alot of time and money into the plane(mostly the airframe) and it's been good so far. Don't forget the paperwork. I paid about $500.00 to have her weighed for a new W&B.
As for other expenses, you will WANT to spend more than you can on airplane stuff

It pays to shop around. Last week I bought a battery that was $105.00 complete including shipping for Aircraft Spruce. It was $175.00 (plus $16.00 for acid) at a local airport...and you can do some of the mantenance yourself. There hasn't been any unexpected expenses so far....knock on wood. 40S lives in the cheapest hangar at the airport and I think it is well worth the extra expense even if I have to drag a couple planes out of the way sometimes to fly. The last owners hangared her, too.