Had no trouble finding it earlier, but I just clicked the Store button in the nav and skipped the product page entirely.

http://www.haystacksoftware.com/arq/ links to ?product=arq3 though, text "per computer for Arq 3".

Please consider integrating or linking http://liangzan.net/aws-glacier-calculator/ (as found in the HN comments somewhere around here), it's been invaluable when talking with people about Arq today.