What is the cost of replacement filters for the: AF 5001 4-stage

Please see the cost of AF-5001 replacement here:
https://www.anchorfilters.com/products/replacement-filter-set-for-4-stage-ro-system-50-gpd

i just bought this thing and the drain keeps draing none stop what do i do to stop it

If you are consuming water, the system will refill it by filtering more water. This process is automatic & continuous, and that is how Reverse Osmosis systems work. Do note that in RO systems, for every 1 gallon of water, about 3-4 gallons is rejected by the system as it contains high amount of dissolved solids. This is normal in any RO system and hence the draining.

What it the ratio of filtered potable water to discharged water: i.e gal/gal

The ratio of filtered to discharged water is about 1:3.

It's like using water to clean water, similar to using water to clean any other thing like utensils, fruits & vegetables, clothes etc.