I have never been to Tokyo Idol Festival, but hellguard has (he lives in Tokyo btw). I'd love to go, but I was planning on going end of May/beginning of June this year.
About flight tickets.
Seems like it's very different prices depending on your country. Prices from Europe start at around 750euro (which is about $1000), unless you want to risk your life on Aeroflot for 600eur or less. If you're only going to Tokyo, I recommend flying into Tokyo directly and not another big city in Japan like Osaka. Shinkansen without Japan Rail Pass costs a bomb and taking a slower train is a waste of time, and you need to be careful with your return flight that it's not too early that you can actually reach the airport in time. (which is why I always stay at least 1 day in the city where I'm taking the flight. In my case, always Tokyo). All the good events are mostly in Tokyo anyway, so I recommend flying into Tokyo (either Narita or Haneda).
About hostels.
I have never stayed in a capsule hotel, but I guess it's dead cheap. However, you have to keep your luggage in a locker which can be a little inconvenient. I've also read that you have to check out every day? (not sure about that).
I usually stay in hostels (lots of choice on
www.hostelworld.com I always book on this site). The most expensive rooms are of course single rooms with bathroom and the cheapest are dorm rooms (sleeps up to 4,8,12 people). I usually go to Japan on my own, so I always get a single room though, but that's personal preference. If you are on a tight budget though, a dorm room is cheapest.
Actual hotels or ryokan are a no-no if you're on a budget of course
Oh yeah, another possibility for the adventurous, is to sleep in a 24h internet cafe
Several of them offer a night package where you pay for 6-9hrs (for less than 1000yen) and some have showers.
About events.
The topic is about TIF, but there's been some questions about Odaiba Gasshuukoku. I have not been to Gasshuukoku but I can tell a bit about it. Odaiba Gasshuukoku is a big event by FujiTV which takes place on Odaiba (where FujiTV is located. Odaiba is actually just a small part of the artificial islands in Tokyo Bay) and spans over several weeks during summer. Idoling!!! usually has a few lives there every week. Last Christmas vacation, they held a winter edition of Gasshuukoku, which I was able to go to. The Idoling!!! lives seemed to be free back then, but I'm not sure about the summer editions.
Solo events are quite difficult, since they're only announced about one month in advance if you're lucky. That's why I make sure to have as many Saturdays & Sundays during my trip, since those are the days they are usually held on.
I should put these tips (and other info) on that wiki page that I mentioned in the other topic. It was originally only meant for finding idol stuff to buy, but I can surely serve for the purpose of event & travel info too