You can be lucky, and you can be unlucky. We run an Arrow III for six group members at £100 a month and £50 an hour, plus the engine accrual. Engine accrual is high because we only started doing it at 1600 hours.
In a good year, I don't have to ask the group members for any more money. In a bad year, it can be £2,500 a person. That was for two new pots and a new prop, which we hope won't happen again for a few years.
Average use is 200 hours a year for the total group, which makes the all-up cost with no surprises very close to £100 an hour.
Add in the "surprises" and you're at £125 an hour, which is about what you'll pay to hire an Arrow III at a club round here. But ours is a much nicer Arrow III.
Of course, if you fly another hour, it only costs you £50 (plus engine acc) because the fixed charges don't go up.
Unless someone's keeping proper books (as I do for our group) it's easy to kid yourself that your flying is a lot cheaper than it really is...
An old friend of mine at work reckoned he flew his Auster for £30 an hour - till I asked him some questions. We ended up at something like £120 an hour. He didn't fly many hours.
It's fine for those who want to kid themselves, I suppose, but misleading for folks contemplating ownership or forming a group and concerned about the real costs.