First of all, it's worth whatever someone is willing to pay, not some random "market number" that you happen to see often in online classifieds.
$100 is a minimal fluctuation in price for any BP morph.. have you considered that the seller you are talking about thought his snake is worth "100 dollars more" simply because it was prettier?
When I set prices, I don't do much "shopping" around for a "market price", I set my prices to whatever I feel my animals are worth.