While amount of money spent on the band is easy to measure, I don't think it accurately portrays who is the most "dedicated" to the band. A poor person who had to save just to buy their albums or go to a show is probably a bigger fan than some upper/middle-class person who doesn't think twice about dropping $300 on a big bunny (or whatever) just to have it.
I think being a big fan is more about how much the band/their music means to you, how devoted you are, etc. Of course this is harder to determine than number of tattoos, possessions, or money spent, but I do think it (devotion, etc.) more in the spirit of being a true fan. Of course, real fans often spend a lot of money on the band, but I don't think it's absolutely necessary.
I also don't think there's one biggest fan.