bcfc IN USA there is something called "The Blue Book" that is very roughly equivelant to Glass's guide to motor vehicles in the UK. Sorry, I do not have contact address for it.
Compared to cars, light aircraft is a very small market, in terms of number of S/H items sold per year.
So yes, you are correct, you really have to look at the ads, and perhaps phone around the companies that do trade in S/H light aircraft, to compare prices.
As you are probably well aware, the main factors affecting a light aircraft's valuation, are general condition (obviously), whether it is on private or public CoA, total airframe hours, engine hours, prop hours, & values of bolt on extras, particularly avionics & autopilots.
A replacement reconditioned engine, with all the extras, for a C172, will cost about £8,000 to £11,000 inc fitting. So clearly a C172 with 20 hours left on the engine, will be worth around £10,000 less than one with a new engine, just fitted. A really good avionics kit & autopilot could have cost up to £10,000.