Its always hard to value vintage cars. The simple fact is that they are only worth what someone is willing to pay for them on the day. Even highly valued antique furniture can have a bad day at the auction house. I've seen NIB untouched Bulldogs going for over $1000, theoretically, as soon as you open the box, you halve that. Personally, in your position (I'm guessing they are in MINT condition with all original parts), if I could sell them for $400 each I'd be happy, but it might be a while before you get any takers even at that price. In any case, you should get enough that you won't have to spend much on a couple of brushless short course trucks for your boys.