I think it's a brilliant idea. Popular estimator sites are making real passive income, in that there's not much work to be done after launching. The snag is this market is saturated and there is a low barrier to entry to keep competitors out. That said, I've never seen a site take an average of the most popular estimator sites. I think it's a clever angle, and it certainly creates the perception of a more accurate result. I'd use your site over your competitors.

Your challenge is going to be in gaining market share. Meaning, you need to rank high in Google! Or spread virally. That'll be tough, but I think you can do it with this angle.