You certainly can, especially in an environment where difficulty increases are being tamed. Here are some factors to consider:

1) juice/hash. You need to know your operating expenses. Figure out exactly what it costs to run the unit each month in electricity. Once you have that number (say $50.00 per month for a 1 TH Dragon) you have a good starting point.

2) expected returns. Look at the unit return in BTC with a 2%, a 5%, a 10%, a 20%, and a 30% average difficulty increase. Find the date for each where at current USD/BTC prices it will be unprofitable to run. Figure out your total profit at each diff if you turned it off that day minus the electricity. This tells you exactly how much you have to spend on the miner assuming each diff.

3) Price of the miner. Lets say you figure out that at 2% diff increases a 1 TH Dragon will net you $1165 by July 2015 after electricity costs. And lets say you buy a used one on ebay, delivered for $399. It does not take a genius to realize that you will over double your money.  Of course, with 30% diff increases you will probably never get the $399 back, you would need to buy the unit for more like $70. Buying new equipment at retail prices will usually get you burned. Look for good second-hand deals, they are out there, not just every single day.

4) Lastly, resale. Believe it or not, people will still buy unprofitable miners. You may well recoup $100 selling your unit when you are done with it. Not everyone can do math.