At the very least, wait until we have more information about what's going to be included. There's no reason to buy it before the first DLC pack is even out, unless you're desparate to have the Liquid Metal weapon skin right now.
More to your point, I'd be inclined to say that there's won't be a price drop on the season pass because the season pass already is a price drop, in a sense. But the L.A. Noire season pass became a deal of the week after a few short months so what do I know. In any case, all you need to do is look at the 400 MSP sale on Gears 2's All Fronts collection to know that Gears 3's DLC will drop in price substantially, it's just a matter of how long you want to wait.