Capsule Hotels are a spezial experience and my experience was ok in all of them. However like stated before they are not so cheap, what makes them expensive is that they are usually very central / close to trainstations.
Another negative point is that you would have to leave in the morning and that you only get a very small locker. And if you get a noisy neigbouhr you might have a short night.
If you want to save money but have your own room to sleep i´d recommend
Economy Backpackers Hotel NEW KOYO
Its not a fancy place and the rooms are very small but 2700yen for a private room is hard to beat in Tokyo. Its only 2 stations to Akiba by train/subway and the stations are around 5 min walk from the hotel. They also feature a japanese style bath, but most foreigners don´t use it (i thought it was quite good).
Internet cafe´s are probably cheaper, but i wouldn´t want to do that for a week. You could crash in 24h Sauna places, there are some scattered around Ueno and i think it was around 2000yen but i´d take the hotel or a youth hostel.