Usually a 4 pack of Frontline XL Dogs cost around $40-45. Mammoth Pet Supplies selling them for $10 because they are short dated and expiry Feb 2014. But if you don't mind the short dating because apparently Australia and U.S. has different opinion about whether Frontline expires or not. In U.S. Frontline does not expire as long as they are kept at room temperature and sealed in its original packaging. Ref: http://www.frontline.com/Pages/FAQ.aspx
See below for more comprehensive price comparison:
http://99petshops.com.au/Search?AnimalCode=&query=Frontline+…
Good price. Shame not 20-40kg though.