I have seen many very presentable 172 and PA28 aircraft go in for a 100 hrly/annual with their new owner and come out $12,000 later. They looked nice on the outside but a good look see turns up corrosion, cracked ribs, wingbolts, control cables etc etc. Then the cash register starts ringing.
A reputable chief engineer told me for a new aircraft that is always hangared, budget $2000 per annual for the first 5 annuals, providing you don't have to replace the control cables at 400hrs like many have had (C172).