I think around 4 is correct. However, now there is really only one that seems to be focused on non-commercial use. There is a fee for the SDK but I don't think it's anything huge. The big problem is that there is a very limited non-commercial market for these apps and not that much money to be made. Certainly you'd need to come out with something at least as good or better than Litchi... which would be _very_ expensive.
I've bought several apps, even thought I don't use them. I also bought Litchi and while I've used it in the past, I don't use it any longer. I have no problem paying $5 or so for a drone app just to support the developer.