What I do is make a want list. Few question to ask yourself is: What type of items do you buy? (ie figures, wallscroll, dvd etc), How often do you buy these items? (weekly, monthly, preorder), Are there any items you missed and you are looking for? If so, how much are you willing to pay for each item at fanime.
Last year, I took $500 with me for purchases, but I spent about $100. One of my friends spent about $120 and the other spent about $40. Their were a lot of tempting items, but somehow we fought it off. This year, probably a different story.