Games might be rare, because they were considered c*ap back in the day (so nobody bought them), or because they were made when the system they were designed for was being phased out, or whatever other reason. They might be common, because they were bundled with a system (even if they were kind of c*ap), or because they were popular back in the day.
Now the demand for a particular game in the used games market is something that is not directly related to the factors above. Usually the game needs to be historically interesting, or a good /popular game back in the day, and have some nostalgy value. Even a game with a high number of sold/manufactured might be expensive - and on the other hand, some software that was too bad to be remembered these days with nostalgy, and that does not have any historical value either whatsoever, might be cheap - even if the number that was ever manufactured is small.
(I don't mean to sound nit-picky here... am I being nit-picky? )