The most common trick, deliberate or accidental, in flight training / leasing in of planes from private individuals, is to "promise" X hours but not achieve it.
A plane that costs £20k will cost rather more than £500 on the average 50hr check - unless it is a parachute with a lawn mower engine on the back
It is not unusual for Annuals to cost £5000+ on planes of this type / age / usage (e.g. a Cessna 150/152). To anybody who can add up, it looks ridiculous but it suits the average PPL training operation which has zero money in the bank and lives totally hand to mouth, hopefully with regular cash advances from punters who pay cash for a 45 hour block in return for a 10% discount.
Remember that no matter how clever you think you are, the flying school owner has been around the block 10 times and is way ahead of you. He's already worked it out.