One of the big differences is the mfg. tolerances, and they are not so strict in the cheaper brands. No billiard ball is perfectly round, and no two are the same exact diameter, but the more expensive ones are much closer to being round, and much closer to being the same size and weight. The materials used also make a difference in how they react when hitting the cue ball and other balls, and how well they stand up to regular play. To answer your question, IMO an average player will notice a significant difference.