Wally, if you compare the $9 per cell price vs an equivalent read-to-use battery, it is way cheaper.
btw, only the Intermec, Jornadas and MobilePros use the 18650 cells. The SL4 uses a different custom batt pack, I think a series of A size li-ion batts in a custom config. When I first refurbed the batt for my previous MobilePro & Compaq M300, the cell is around $8~$9. If its still around that price, I say its didn't changed much.
If you are willing to buy 2000mah batts instead of the newer 2400mah, I think they go for $6 or so.