It is zone-based. If you are close enough to a plaza, you won't be tolled getting on, if you're just after a plaza, you won't be tolled getting off. The toll for each ramp is more expensive the further it is from a plaza. It's possible for you to hit only one ramp toll in a short run, but if you were to math the total paid on any given route it will work out to be roughly linear with your distance.