The best way to determine the value of an aircraft is to track what your looking for in publications such as Trade a Plane.
This will give you a feel for what the market is on your individual choice of aircraft.(remember nothing is worth more than someone else will pay for it.) Next get very familiar with AD's and maintenence, talk to mechanics that specialize in the type of aircraft that you are considering. This will give you a feel for what you can expect in future costs, which in my opinion is a big factor in the price consideration. Dont let price guideline books or valuation services weigh to heavy on determining what youll pay.( how many times have you seen some rednecks truck in a classified with a 10" lift kit stating "blue book cost $50.000.00")
Trust me those books and services are no substitute for doing your homework. If you plan on financing your aircraft usually your lending service will let you use a mechanic familiar with the aircraft to write an estimate. My rule of thumb when purchasing aircraft is to know what it is worth within a $100.00 just by looking at the spec sheet and verifying that with a thorough buyers inspection. Trust me, if you really do your homework you wont need a valuation service or "blue book".