When you look at the price of the batteries at around £2 each, you realise that the battery packs at £50 and up are something of a rip-off. I'm fairly certain that the circuit protection board, cable and shrink wrap that constitutes the rest of the battery pack would cost no more than a fiver.
Having said all that, I wonder if there really is any circuit protection in the battery pack itself. It seems that this may actually be built into the battery:
http://www.ebay.co.uk/itm/4x-UltraF...rElectronics_Batteries_SM&hash=item336fcf6b60
And any overcharge protection will likely be built into the charger.
I wouldn't mind betting that most of the battery packs are simply four 18650 batteries with a suitable cable attached in a heatshrink or waterproof case: