I've been on the fence regarding an ASIC miner purchase, too. I've run a ton of calculations based on various estimates of difficulty in the next 4-5 months. Unfortunately, it's very hard to know what is a "best case" scenario or "worst case" since there's so much going on in the ASIC realm right now and "soon."
I've run projections around 15-30 times the current difficulty just to make sure I'm thinking critically about where all these new ASIC miners will be taking things. There are several companies promising great products, but not much is proven yet.
I put a lot of mileage into this calculator to help think through it:
https://bitclockers.com/calcJust remember to run estimates down the road a ways and not just based on when you first get it. Calcs like the one linked help to estimate taking monthly diff increase into account.