They are price guides they get their prices from Auction sales of comparable vehicles. Most prices that the general public can see are over in falted BS prices.
Most prices on Collector cars any more is what the market will bear.
Most people don't even know what a #1 condition car is. It is a fully restored /date correct down to every nut and bolt , never driven trailer queen. That has more money put into than they will ever think of getting back out it.
What type of dart are you talking about are you looking at big block GTS Dart or 340 4 speed GTS ?