Muahahaha. There are insane stunt jumps, and then there are INSANE stunt jumps...
There are twenty Unique Stunt Jumps scattered around Liberty City (eight in Portland, though some are somewhat inaccessible at first). When you hit one of those, the camera goes slo-mo and you get a big cash bonus on your first successful landing. As a free hint, the first one is near your Portland hideout -- look for a concrete wedge near the water and the overhead railway tracks. Look at the long straightaway leading up to that concrete wedge. Look at how the wedge points up and over the railway bridge. Do the math.
As for your garden-variety insane stunts, there are criteria for jump distance, jump rotation and flips. If you surpass one of them, you get an Insane Stunt Bonus. Two, and you get a Double Insane Stunt Bonus; three gets a Triple. If you land squarely, you can get a Perfect (1,2,3x) Insane Stunt Bonus, which adds to the bounty somewhat.
The game can be picky as to what constitutes a perfect landing (or even when it'll give you an insane bonus at all), but vehicular mayhem is half the fun of the game -- the vehicles are generally quite replaceable, so it's a blast just to drive around and see how horrendously you can jump and crash them.
Now, the MOTHER OF ALL INSANE JUMPS is extremely difficult to pull off, and even more difficult to survive without cheating, but it's worth seeing at least once. There's a jump in Chinatown that can put your car up on the overhead railway tracks. If you get there in one piece, you can maneuver your car to a certain position and wait for the train to hit it. Sometimes you're just knocked off the tracks, sometimes you're wedged INTO the tracks... but once in a blue moon, the train hits you JUST right and your car is launched into the sky, spinning like a top. If you aim it right, you can get hit in Portland and land in Staunton Island!