I'm trying to figure out the math to get estimations on which is more profitable, buying GPU and mine alt-coins with multi coin pools? or buying Antminer S9 and mining BTC.
I've did some math for the BTC part, but I'm struggling on finding a formula to calculate GPU mining earnings.
Anyone knows how to get rough estimations for it??
Profit-switching techniques often use lead to mining coins that are too volatile for justifiable estimates to be made, even after fudge factoring some percentage off.
I would suggest calculating your GPU estimate with something *relatively* stable, such as ZEC or ETH (depending on your GPU selections), then if your BTC ASIC estimate still comes out as possibly more profitable, then come back. (If anything, profit-switching should be slightly more profitable than the projection provided by ZEC/ETH, so if the S9 can't beat that, the GPUs may be a better bet.)
I assume you've also considered other aspects of the GPU vs. ASIC debate (resale value, availability, maintenance, etc.), so I won't get into the details of that here.