SLB510 attys are often cheaper than Joye510s. They are also higher resistance, ~3.0 ohms vs ~2.4 ohms. These are actually quite good on higher voltage PVs, but tend to be too weak for regular or mega 510 batts (~3.1V with load). I have gotten SLBs for as little as $4 a piece, can't remember where, and they are very nice on my 5V PT.
If the 510 attys being sold are cheaper than $6, check to see if they are Joye, or if a resistance is given.