I can't tell you about a C150, but I bought a 1961 C172 in Oct 2009, so I am just just completing 2 years ownership. I paid £17k for it, and had to spend about £2500 on initial repairs, (I knew it needed a new cylinder and various other bits and pieces so that was figured into the price). The engine is lowish hours, but more than 12 years since major overhaul so running on condition, (hence I could not lease it to a school even if I wanted to).
Fixed (ish) costs...Hangarage, (expensive S/E England) is £300 pcm (outside parking would be half), Insurance c. 100hrs/year, £995, Planned maintenance (annual and 1 x 50 hr check each year), has been about £8k/year (mainly because, even though "planned," unexpected things turn up at routine checks in old machines). Unplanned maintenance, i.e. things have gone wrong and needed fixing, have cost me another £1700.
Variable costs.....Avgas 36lph wheels up to wheels down. Oil, 1 lt every 7 or 8hours, landing fees, (lots).
If it was a 150, I would imagine that having 2/3 of the cylinders it would probably have cost about 2/3 of the maintenance and used about 2/3 of the fuel.
Total cost per hour, wet, including VAT, but excluding landing fees has been £196 per hour chock to chock.
I am sure that if I had bought a much newer machine I would have had to spend a lot less on maintenance, but if this aircraft needs a new engine, I can scrap the airframe, walk away, and not lose more than the 17K I paid for it.
I wanted a Cof A machine as I have a night and IMC rating and do occasionally use them, particularly as I use the aircraft regularly to get myself to work, and want as much flexibility as possible. If I did not, I would have gone for a permit machine.
Hope that helps.