The online estimators are designed and used by the insurance industry, and are notorious for undervaluing an airplane. Both of my airplanes came in more than $5K below what I considered to be their true value. I believe I've posted about this in the past, but the reader's digest version is that the insurance industry would rather guess a little low than high, and they are compensating for owners who typically would rather guess high than low.
It's my observation that rather than setting their values a certain percentage low, which seems more logical, they knock off a portion instead, of between $5K - $15K off the top. This is not as big an issue on a $150K airplane like a Mooney, Bonanza or 182, but knocks as much as 25% of the value off a C150-152. Where it really comes into play is when you sign up for insurance, and they don't want to insure the airplane for what you paid for it.
