This model works for me...
One sees approx 5000 serious flight simmers worldwide
1) Pay for for base sim - 30-50 USD = 250,000 USD
2) Pay per plane - 5-10 USD = 50,000 USD
3) Maps are for free.
So if you end up with an IL2 type sim with just 100 or so aircraft sold
Base Sim = 250,000
100 aircraft = 5,000,000
TOTAL (approx min) = 5,250,000 USD
It makes more sense to cover your basic cost (base sim) in case the game flops.. then a/c sold is more profitable.