Expensive? I don't think that 16$50 for a cell is too much if you need 2 or 4 of them (2s2p). That makes 66$ for the 2s2p 4600mAh 280g 6.6V nominal voltage pack. Advantages over LiPo are huge - very high charging current (can be charged in 15 minutes), hardcase, won't burst into flames, can handle low-voltage discarge (even to 0V and still ok). These are the "deWalt 36V" batteries or A123-s. They only have a bit worse Wh/kg ratio than LiPos, but better than NiMH and much more user friendly than both. Most newer chargers already know how to charge them.
http://media.hyperion.hk/dn/a123/123-brief.pdf
http://www.aircraft-world.com/shopdi...asp?search=yes
Even our local small battery-shop is selling them. In white "no-name" casing like the Hyperion one but without the logo. OEM from A123 I think.