Charge a nominal fee like $2 to park at metro on the weekends to raise money.
There is no reason to give away services for free and a parking lot is a service. You charge for it during the week, so charge for it on the weekend but less. It raises funds that can be used for system pr other improvements that are very much needed. Plus a nominal cost, like $2 per day, isn't a big deal. People pay more for coffee.