Cost to go to 128" will only be the cost of the rail extensions, since you don't have to stretch your tunnel. I've never bought only rail extension but I would guess probably about 130 bucks.
The predator is a good track, very good track. If you are looking for top speed I agree with sledfreak, and get the 1" predator. Tracksusa.com has them on sale right now for dirt cheap.
If you are looking for more traction for acceleration and braking, and some off trail performance, but don't care about top speed (if 100mph is enough), then go for the sno pro track.
I wouldn't go to 128", I can't see 3 extra inches on the ground doing a whole lot. Plus you are very limited in your track choices. Basically limited to the Cobra or a prestudded track. And they are all junk if you ask me, cobra included. Never seen a Predator in 128" length either.