My adult child lives in an area of Seattle where parking minimums have been eliminated. Their building, built 3 years ago, has about 65 adult residents. There are 10 parking spaces on site and the street the building sits on has zero street parking. Renting a parking space adds about 10% to the cost of renting. Currently there is still one spot available if a resident wants to pay for it.

If they had built 60 parking spaces and let residents park for free, I feel confident there would be at least 50 spots in regular use based on how nearby buildings with larger parking lots are utilized. Since the parking is underground, the most expensive type of parking to build, I also feel confident that rents would be at least 10% and probably 20% higher with "free" parking.