I built my own RO system it cost around $1500 including a water softener. There are some ongoing supplies every year, maybe $200 or less.

I don't have any links I just figured it out, but it's not super complicated. I made it out of undersink RO membrane housings (housings from those little RO systems you can buy for around $300 that do a couple of gallons a day). The membranes have pressure pumps in front of them that get it up to a couple hundred gallons RO water total a day.

Basic steps are 1) Soften the water, 2) Pass it through very tight filters (like 1 micron), I also carbon filter for organic contaminants, 3) Booster pumps put water through osmosis membranes and from there into a storage tank.

I just used plastic totes with gravel in the bottom to house the membranes and booster pumps.

I should write up a blog post on it one day because professionally installed osmosis can be expensive.