Good quality inserts such as the premiership commemorative cards and DPSs of decent players will always do okay, but they are not going to make anyone rich.
Most Teamcoach and Select inserts halve in price within 12 months if you are prepared to be patient. I am amazed at the prices that people pay for Firepower cards and the like, which have no significance whatsoever. At least with B&F or rising star inserts you know that the cards represent some sort of achievement, even if they are fairly easy to come by.