Wow, I was a little blown away by your post, tango. It reminds me so much of me, how I was around the time I also wanted to go to her concert. Those were the days, wicked.
Yokohama Arena official English page:
http://www.yokohama-arena.co.jp/english/index.html
As for auction prices, you can do a couple of searches on auctions.yahoo.co.jp, and you'll get some clues. Or you can go with Hatix's estimations.
Your excitement made my morning, tango, I'm glad this thread really does work in the way I was hoping for it to. To help out and inspire more people to be able to realize their dream.