Bearing in mind that this is still in the territory of "leaked before actually announced", and as much as they're now closer to San Jose than they are to Oakland (at least both in my memory and on the map, pending a dictatorial ruling), but the bottom line simply seems to be that this was the best real estate deal they could grind out.
But in the long term it could work well for them if it means they can draw more of the San Jose populous than they are already, plus the fact that it'll be a shiny new balpark, which usually draws well the first few seasons.
I guess you'd be surprised at what teams will do when they're contending and need to make a trade at the deadline. The Dodgers picked up Jeromy Burnitz last year for god's sake. Burnitz was paid $12 million last season.