Simply pull a few drifts when the game says you've made an "epic collateral" stunt, or something like that, and then start smashing fences again. You have to get 100 Bucks by smashing things in a single stunt combo, not in one go.
I just had to do 200 B collateral in a single combo. The easiest way was to find a stretch of highway and knock over the light poles in the middle, occasionally going a bit to the left or right to collect oncoming and overtake points before going back to the center.
I always use the west side of the miltary base (north of Chicago) for those collateral challenges. The trouble with the Mexican border is that it's a lot of up and down and not that many long straight sections, so it's easy to accidently miss a piece of fence.
Or do some pavement City driving and scare some pedestrians at the same time. You might end up in a Police chase and do a couple of achievements at once.
PS Those long fences come in handy as one of the achievements is to smash 1.000.000 objects..
Makes you wonder what dev came up with that idea