Thanks for the reply, I am definitely missing something.
My impression is that you have your math correct. It's just that every possible variable, except for your investment(s), isn't a static number in the whole. Hence, asking for somewhat solid numbers was kinda hard, since the whole tends to change every time one of those variables change
I notice that prices have gone up in the last 24 hours. Is this because of the BTC continuing drop?
Prices also went up when the BTC was in it's bull run lately. So more probable, it's due to people simply buying up S7 hashes again.
The other thing to remember is that the markets are not rational. They don't behave like traditional markets and any attempts to make correlations between btc price or difficulty compared to price of hashes may not be relevant at all.