what I would do was take about £500 with me for travel food and accomodation over the course of a week (including a drinking session in Bar 23 in the Asakusa area) , give or take, but im sure you could get away with a bit less if moneys a bit tight.
Iv been to akihabara a couple of times, its amazing! id never seen so much anime before (or since) than in those stores!
Have a great time