I am right there with you about values found online. Lets say there was a beautiful double wide on a one acre parcel and right next door a 2010 frame built home with pool. If the lots same in size and interior footage exact same, than the online probably are identical. So if you have a tract one home has all the upgrades and the other bought no upgrades the online pricing will be incorrect. The one who calculated the formulas did not consider the age of the home and quality. Its simply a mathematical equation, which is why lenders and realtors still need to be in person to give correct numbers. The AVM method (which lenders are now using includes more data) but here again if your subject is newly constructed or simply stated is just far better than its comps well that number will likely be low. Also consider if that local market is in flux, either moving up or down in a short period of time, online numbers will be incorrect.