There is always a bottom for every coin, and there are parts that can be calculated, a mining rig costs money to buy, running a mining rig consumes electricity which also costs money in most cases. I have made a calculation which can help you to understand the factors that CAN be calculated, the only one that is always an unknown is the human factor. I am using 20GH/s or 20000MH/s as networkhashrate, but you can change that variable as needed, the same applies to your minerspeed. I am working with euro values but you can change all to dollars and get the same results.
NLG coins per day = 576000
Electricity costs per KW/h in the Netherlands = 0,23 (eurocent)
Networkhashrate = 20000MH/s
Result per MH/s per day = 28.8 NLG (576000/20000)
Let's say you have a Zeus X6 running at around 44MH/s and consuming 1000Watt, that means it will eat 24KW/h per day at 0,23 per KW/h which is 5,52 in electricity costs per day. Your miner produces 44 x 28.8 NLG = 1267.2 NLG per day, so the costs per NLG coin is 0.004356. The current BTC value in euro is around 370 euro, if you divide 370 / 0.004356 the result = 84940 , which gives you the number of NLG per BTC, now divide 1 / 84940 = 0.00001177 and you have your bottomprice where you break even, of course you want to make a profit and if possible also earn back your investment in your mining rig, so you will have to sell it for quite a bit more to actually make a profit.
I hope this helps a bit and feel free to correct me if my calculation is off