The big thing is how frequently the thing is flying. I've seen engines 500 hrs since overhaul that have been sitting idle for five years and quit within 50 hrs and I purchased a C172 for my flight school 200 hrs from TBO that we put 600 more hours on with no problems whatsoever (flying 80 hrs a month). Standard calculated assumption is that an aircraft will fly 40 hrs a month since production. So you need to multiply out when the engine should have been overhauled from a time standpoint when calculating value as well.