The best money ever spent would have to be the $1600 (purchase price) for a 1988 volvo station wagon with 190k miles on it, this was about 2.5 yrs ago. At the time I thought we were making a bad decision buying a car with that many miles on it but the wife insisted that she likes dorky wagons...not only did it turn out to be reliable but also an extremely practical gas saver. We've loaded it up to the gills on our Home Depot trips many times and it still gave us 22mpg. We took it down to Baja, Mexico in the no man's land and it never complained about the 2 day 700 mile trip. At 240k miles on it its still running well.
The most beneficial part of the ownership has been the huge improvement in my knowledge of working on cars. I'm not afraid to tear into it when needed. In fact, now after 2.5 yrs of wrenching on the volvo I'm not afraid to work on our other complex/expensive cars either...
Now watch it self destruct when I go outside to give it a pat... 