One of the hardest things to learn upon becoming an aircraft owner is that so few businesses (and individuals) working in GA can be trusted.
The % is below any other business I've been remotely involved in, and this probably includes double glazing, central heating, you name it.
It starts with the dealer who sold you the plane (a lot of dealers are quite sharp) and you move on from there.
I think a part of the problem is that a lot of people are not making much money and, when they are busy and sending out invoices, they aren't getting paid. Another part is that aviation has a somewhat romantic air to it and thus attracts a lot of "character" types; the sort that dominate the airport bar with amazing stories but when it comes to something concrete they usually cannot be trusted as far as one can throw them (which isn't very far if they spend a lot of time in the airport bar).
A lot of people are awfully nice but simply clueless. My
universal experience with a number of avionics outfits, even some high recommended and well known ones, is that they know very slightly more about aircraft electronics than your garage knows about car electronics (you follow the wiring diagram provided and if it doesn't work you give up).
Anyway, to answer your question
the way to do this is to casually and in a totally non-committal way walk in to the office and ask for standard pricings for various types of service, for a particular aircraft type. Get it written down and keep it.
The cost of a 50hr check for a new C172 will be similar to a 30 year old C172, but whereas the new one will cost say £200 plus £40 oil plus very little else, the 30 year old one could be £200 plus £40 oil plus £1000 for fixing things which have been discovered.
An Annual for a complex SEP should be about £1500+VAT. Or about £1800+VAT if it is N-reg
Hourly rates in GA maintenance are a lot lower than car maintenance. This is curious, and is perhaps the result of a competitive climate, dominated by customers willing to fly 50-100nm to get a cheaper (or perhaps "less fussy"??
) 50hr check, and refusing to pay for anything if the bill comes to too much, which it often does due to the very high cost of some parts. This travelling for an away-service seems to be particularly prevalent among schools, operating under Transport CofA.