Actually, if you already have the tub and a good set of filters, I would only buy four Tri-X. Keep a regular filter on the Circ pump. The regular filters would just about last the rest of your life if you either rotate them or use one till it 'won't go no more...'
The Tri-x filters range in price - some dealers apply a standard markup while others simply sell them as a convienient way to have something others don't have. I think it depends upon the issue of how they get them: they cost a lot to ship UPS or FedEx, but if you only get them with a spa order you tend to have to buy dozens at a time in order to be able to sell them, since they go out the door in threes, fours and fives.
