I notice that Hobbyking now has 10,000 mah 4S batteries for £26.57. 4S means 14.8 volts nominal, 16.8 volts fully charged. I use them as three in series (12S), giving me 50 volts fully charged. Three of them would weigh 2.4 kg and cost £80, and I'd expect to get an whole morning or afternoon caching from a set of three, and I'm very heavy on the power usage.
Before I found out about Lipo, I was paying about £200 for 7S of 10,000 mah, which is more than twice as much cash for a bit more than half the power. These "Multistar" batteries are now the most cost-effective batteries I've found, even better than the 5000 mah 4S "Hardcase" batteries that have half the power and cost a bit more than half the cash.
So I've just bought a set of three, together with a new 80W charger (the one I got from Ebay just stopped working) and a set of three 30W chargers that might work out more effective.