50 lbs of boric acid from The Chemistry Store delivered to Maryland is $107. The best price I saw for ProTeam Supreme Plus on eBay was $149 delivered for 45 lbs. That makes boric acid from The Chemistry Store $2.14/pound and ProTeam Supreme from eBay $3.31/pound, which isn't even close.
Using California, instead of Maryland, the shipping at The Chemistry Store goes up substantially, for a total of $131 delivered. The eBay price remains the same. At $2.62/pound for boric acid, it is still less than ProTeam Supreme Plus at eBay. Alternatively, you could get boric acid from
AAA Chemicals, which is cheaper with shipping to California (though more to Maryland). Their total is $116 delivered, or $2.32/pound.
Perhaps you were accidentally looking at ProTeam Supreme, which requires lots of muriatic acid, just like Borax does. It is about $105 for 45 lbs delivered on eBay. That is $2.33/pound, but that doesn't count the acid, which will make it significantly more expensive before you are done.
The calculations for smaller sizes are somewhat different, though The Chemistry Store remains less expensive for most sizes shipped to most locations.