I always thought the new Visual Studio pricing model was a good way to go for this: Free for teams of <=5, cost/seat for teams above that. I suppose if you go

I always thought the new Visual Studio pricing model was a good way to go for this: Free for teams of <=5, cost/seat for teams above that. I suppose if you go that route you need a legal team that can afford to be on the lookout for violators though.