What battery pack to get

I am thinking of creating a portable raspberry pi of some kind and the only thing that stopped me was the battery pack.

The screen I will be using is below

http://www.amazon.co.uk/inch-Digital-Rear-Monitor-Square/dp/B0056WJP1M/ref=pd_sim_computers_6

and of course the raspberry pi. So I need something that can power both of these things and at a reasonable price. Thanks.