Optimum level for upgrading Opera House?

I've just done a quick calculation, based on playing for a couple of hours in the morning and 6 in the evening which is what I'm managing at the moment during the week, of the payback times for levelling up. Based on this I can recoup the expense of levelling up from 8 to 9 in 7.8 days from the extra masks generated but levelling up from 9 to 10 would take 16.2 days to pay back. Unless you play a lot more often then I or have a lot more characters and I have a fair few, level 9 is likely to be the best bet.