LIke what, 10%, 5%, 2% or .000001?
Probably somewhere between 0.001% and 1%, if the police were stopping people at random times and places. But who other than you has suggested that police are doing that? I would guess that there are some places where, if some cops with binoculars were to stop all of the cars which entered the state after the occupant emerged from a store that was within sight of the border carrying a decent-sized bag, 5% or more of the cars stopped would be found to have some form of contraband (most likely an unlawful number of out-of-state cigarettes). The fact that someone was seen leaving a store where they could just as likely have bought legal goods as illegal ones should not be considered sufficient probable cause to justify searching that person's car, but contraband would be found in enough cars to make such efforts worthwhile (at least from the point of view of the cops).