Eh, I dont know, for my 18'x52" round pool, needed I think six or seven 40lb bags to start, if I kept the pool filled, that of course would not be an annual expense, but even if it is, $6 a bag x 7 bags aint a whole lot, considering you most likely dont have to buy any more during the year.
Walmart.com: Morton Salt Pool Salt, 40 Lb Bag: Outdoor Play