Last year's Mystery trip went to Malaysia, and the year before was New Zealand. And cost is no way to try and guess where this year's trip is. All the KWEST trips are required to charge within a certain range so that cost is less of a factor in choosing a trip (I think the range is like $1800-$2300). So even a trip to Mexico or the Carribean ends up costing about the same as a trip to Asia, with the tradeoff being the trips with cheaper airfare have more to spend on accomodations. I think last year the Puerto Rico trip stayed in the Ritz.
Good luck trying to guess where Mystery is headed this year, it's one of the most closely guarded secrets at Kellogg. It's definitely worth it though, the Mystery group is very close with each other.