If he's really that yellow in person - I'd pay up to $400 for him. But that's just me - I'm willing to pay Grade A prices for a Grade A animal, and not pay Grade B and C prices.
Someone in Daytona offered $1500 for my adult male lemon pastel, Winston when he was on the BP.net table. Of course he wasn't for sale, so he's still in my collection.
With people breeding pastels just to breed them, and the plethora of average looking pastels, nice ones are worth far more to me, personally.