Where do you draw the line if your DC wants to participate in too many and/or too high level competitions?

I am talking about math competitions in particular but the question applies to other types of competitions as well. By "too high level," I am talking about competitions where your DC can likely manage to score respectably (say, top quarter or even top 10%) but not at the awards level. DS11 has expressed interest in participating in a number of local/regional high school level math competitions in addition to AMC10, in which he will participate again this year. My initial plan would have been to wait until he is actually competitive so he has a real shot at the prizes or a year or two prior. However, I have agreed to one local one for next fall due to the significant prizes attached and the idea that practice/experience can be helpful down the road. Now there are other ones, including one next month that we will need to decide soon.