It's probably going to run about $20-$25 a bottle no matter where you get it. I would check the local pool stores and possible hardware stores (Ace Hardware in my area carries it). Often, it's cheaper to buy it locally once you figure in shipping charges.
Do you specifically need to use the purple stuff for warranty? If not and you just need to sequster metals you can use any seqesterant that is HEDP based, even with a SWG. I use the Ace house brand O-Ace-Sis and Proteam Metal Magic pretty much interchangeably and both are less expensive than Jacks.