Help and Advice

Ok before you all start, I know I'm lazy....I go to my LFS once every two weeks and buy 5 gallons of ready mixed pro reef salt water and throw it in, I've had no problems what so ever doing it this way, it cost £6.50 a time, that's £169 a year, and is so easy.
My question is what would it cost to buy the pro reef salt myself and use my own RO water over 12 months including one RO filter change a year if that's enough?
I'm excluding the cost of buying the RO unit as I...