6 x Alkaline AA's = 9volts, average 6 cases of paint, weight approx. 7 oz
2 x Alkaline 9V = 9.4volts, average 3 cases of paint, weight approx. 3.5 oz
price
pack of 10 AA's procells = £2.20 (3 packs = average 30 cases of paint) Total £6.60
Pack of 10 9v procells = £8.80 (2 packs = average 30 cases of paint) Total £17.60
Or you could get yourself a rechargeable battery pack (specifically made for paintball hoppers), some for as little as £17 (that would last you 7+ cases of paint on one charge), some others costing over £50 (which can last for around 20 cases of paint on one charge). Never having to worry batteries dying in a middle of a game.
Another advantage of a rechargeable battery pack would be the cost in the long run. This depends how much time you play and how much paint you shoot.
Theres also the option to buy rechargeable AA's and 9/9.6v
http://www.mahaenergy.co.uk/
The 2500mah AA's would give you 10+ cases easily with the one charge. As with any rechargeable batteries and packs, they loose there charge over time if left unused. But theres a great advantage of having these. When not paintballing, they are useful with other gear that uses batteries.
If you stick with alkaline batteries, make sure to switch hopper off in between games, and take batteries out of hopper after a days play (even when hopper is not switch on, the hopper will still drain a small current from the batteries if left connected in your hopper)