I think another key point the op made was that "a couple of the pro riders' sponsors have been asking to buy my shots". So would be be a good idea to just ask them what their budget is? If they are a serious purchaser of photos they will have a figure in mind, if it is too low just reject it or get them to go higher. Also ask if it was just that one photo they like or is it your work they like? You could do a deal, like x number of photos for a much larger price then just 1 photo.