Some rules to live by:
No battery is going to last forever. They all have a limited number of charge/discharge cycles.
Cheap batteries, just like cheap engines, aren't going to last as well as good quality ones.
Cheap batteries also tend to suffer from spec inflation -- basically they'll claim a higher capacity just by moving the goal posts as far as what's altogether charged or altogether discharged.
LiPo cells, specifically, don't like to be over charged, or discharged too deep, and they don't like to spend too much time at either end of the charge curve. Which is why folks tend to obsess over the whole "storage charge" thing.
So, whatever you get, the more you pamper it the longer it'll last, and you may want to pamper the cheap cells a bit more. The closer you get to the ThunderPower end of the spectrum, the longer it will last.
50 cycles for Turnigy cells is kind of on the low side for battery life, but it's not absolutely horrible.