I think it depends on the set. I remember FutureSight prerelease where they were selling Tarmogoyf at really cheap prices. Invest in a card that you think would be the cornerstone of a championship deck and boom watch it's price soar +_+ I remember the blue planeswalker sold high at the prerelease but its utility pretty much let it either stay at that price or go lower a bit ._.
I think its usually a tourney pack and 2~3 boosters for the entrance fee in the prerelease.
Ive heard from my friends that blue white might dominate incoming sets. Also, if you want spoilers of the incoming cards in Eventide: Eventide Spoiler 150/180