I feel lucky the highest I've seen my TDS going in is 23 and my normal water pressure from my washer "Y" is 90 PSI at my RO/DI unit. I haven't changed my filters in prob over a year, I really should check my membrane tho, TDS coming out is still 0. I refill my 5 gal top off every 3-4 days and around 500 gallons in WCs, so roughly 1000 gallons a year.
My LFS charges $.50/gal for RO/DI
$1/gal for Mixed Saltwater
I bought the unit used and put all new filters and membrane in it, about $100 total.
I also rent my place so I don't pay for water
So I've save about $400 with my RO/DI unit, and that's just in FW.