baseballbrat,
You should answer for which round your son would sign in. You can give a range, if you wish, rounds 5-10 as an example.
If it is about the bonus money, you first need to search for last year's bonus slots and determine what it would take for him to sign, to forgo college. If he is not concerned about the bonus money, then it doesn't matter.
If asked about how MUCH $$ would it take to sign, the best answer would be fair market value. As an example, fair market value for a HS, RHP throwing 92 committed to such and such school. You can search scouting reports and compare where HS RHP throwing that veocity might have signed for last year. This would be done taking into consideration your son's position. You can also determine by where your son fits in the overall scheme of his HS year (rankings) and the state (rankings) you reside in. It takes a bit of work, but will give you a better understanding of the draft and how it works.
You need to be realistic in your approach.
I have tried to explain as easily as possible what you should do to answer that question, if your son is serious about signing. It is early yet, if interested you should have them in by the time season comes around.