If the police were stopping cars at random times and places, the probability would be pretty low. On the other hand, there are some places and times where the percentage of vehicles carrying significant amounts of cash would be much higher. If a cop wanted to target vehicles carrying cash, it would not be hard to identify likely candidates.
Further, even a cop who actually wanted to catch "criminals" could probably identify cars that were more likely than most (though by enough to constitute "probable cause") to contain something illegal, even if not something a dog would detect. For example, if a cop observes people visiting a store just outside Illinois which among other things sold cheap cigarettes, a search of all such people would probably find a number that were carrying an illegal quantity of cigarettes (I think the legal limit is one or two cases). Merely visiting the store and carrying a bag out of it wouldn't constitute probable cause (since someone could have bought other merchandise and/or a quantity of cigarettes that complies with the legal limit), and there's no way a dog could distinguish Illinois-taxed cigarettes from out-of-state cigarettes nor determine the exact quantity of out-of-state cigarettes, but a cop who used drug dogs as a pretext for searches could probably net quite a few "cigarette bootleggers".
LIke what, 10%, 5%, 2% or .000001?