As long as you go into it with the understanding that you are not going to make back whatever you spent on the device - not placing any value on any "fun" or learning you would derive from the purchase - go hog wild !
I don't know where you get the statement that you won't make anything back....IF mining were not profitable NO ONE would do it. Now granted you shouldn't go into it if you pay crazy rates for power, but if your power rate is reasonable then you most certainly can make money mining. Accounting for the diff going up at $25k per unit the SP50 would pay for itself it in less than 9 months. Everything after that is profit. Even if the diff shot WAY WAY up so what. Instead of taking 9 months to ROI it takes 12 months. The point is, as with any investment, you have to be patient.