If its just the hours you're after, it is difficult to find anything cheaper than a PFA permit "group A" aeroplane. I am treasurer of a Jodel group flying on a PFA permit. The group has operated for about 20 years and I like to think our group is extremely well run. To give you some idea of costs; share price (1/6 share) £2500, monthly standing charge is £37, cost per hour (airborne, not brakes 0ff/On) £6 per hour plus you buy your own AVGAS and leave the aircraft full for the next flight.
There are several commercial pilots flying now ( including me), who built hours (used to need 700 hours) on this aircraft.
Downside:
No electrics, so you have to swing the prop. This is more an imagined problem than a real problem.
PFA permit, so no IFR or night flights allowed.
Only 3 hour-to-dry-tanks range, and only 80 knot cruise, so route of a long trip is often determined by fuel availibility.
Like many interesting aircraft it is a taildragger, so you have to do some more learning.
I now fly for a living, but wouldn't have got my current job without all the tailwheel hours, the bulk of which I got on this aircraft.
Look around there are plenty of groups. Like everything in aviation, its still a financial risk, but compared to the cost of commercial training the risk is modest.