If you have a small time FBO in the area that rents 150/152s you can often figure they're just about breaking even on them (in a good year). This is one way to quage the price of ownership.
Otherwise you can sit down and run the figures on the cost of fuel, oil, insurance, tiedown/hangar, financing costs (if applicable),etc. It costs approximately $5 per hour for a major overhaul (if all goes well), but over the course of the engine's lifetime there may be the once-in-awhile repair needed...or the dreaded "Oh crap! Not again" problems that seem to plague some aircraft. Other possible considerations would be repair/replacement of avionics, instruments, electrical, airframe, tires, etc. etc. ad nausium.
Sound bleak? Well it isn't...not unless you inadvertantly buy a lemon. By and large the 150/152 is dependable and affordable.
There is no difinitive answer to your question because there are too many variables and no guarantees. If I had to financially justify owning N150DM I'm afraid I'd have to put her on eBay tomorrow. Most folks own small airplanes because they want to, not because it pencils out on a spread sheat.
Best of luck with your decision making <img src="/ubbthreads/images/graemlins/tongue.gif" alt="" />
