I've consistently spent about £15 a month personally. On a rare occasion more, I've topped up £30 in a month once or twice. I guess I'm the person more suited to a contract just don't like the 18month lock in
Its pretty expensive then, but that is at least unlocked.
Locked on PAYG I'd expect a small subsidy (up to £50) and no profit to be made on the phone itself. Whereas unlocked the phone itself has to make a profit.