I run Turnigy Nano-Tec 5300mah hardcase packs and can run 20min, so I would look at the smaller 5.3 or 6.0 packs and get them if in stick, which they normally are and are cheaper since everyone wants the 6.6 packs.

On cost, look at it this way, you spend it up front on the kit or in back on parts. As long as you don't have to spend it on both sides you are good. From what I have read and expierenced in the short time I have ran Serpent is that quality and durability is Superpub.