Here's my calculation. Check out my google spreadsheet
I looked at the Ethereum mining participation one-month after launch to get an idea for what the ZEC mining participation will be. It's about 350 GH/S.
I figured everybody was using 290x GPUs. They get 22 MH/s according to here
This puts the network at about 16,000 290xs one-month in.
In ZEC, a 290x is going to get something like 25 H/s, so figure about 400,000 H/s for the whole network.
Depending on your assumptions about ZCash price, that gives different amounts of return for each H/s you own. At a ZEC price of $10, it returns about $5 a month. At a $100 price, it is about $50. During the first month your profitability will be halved on average due to slow-start.
Toomim and Genesis are charging about $100 for 1 H/s for a year. Your break-even ZEC price is about $10 if the mining participation is similar to Ethereum's.
At $100/ZEC, your ROI is 50% per month. Not bad! But of course this kind of ROI is likely to attract more miners and squeeze it down.