From Minneapolis / St. Paul Business Journal: Metro Transit cuts two park and rides
One was a 288-stall lot, from the article "Metro Transit, a metropolitan-wide government agency, expects to save $17,000 in plowing and $16,000 in lighting expense for the surface lot per year, Gibbons said." So $33,000 per 288 spaces, assuming all were occupied, amounts to a subsidy of $114.58 per parked car per year. (Divide by load factor for the actual subsidy which is higher), or at least $0.45 per day per car. This adds to the cross-subsidy from people who walk-to-transit to people who drive-to-transit. Given transit fares are $2.25 (local) in rush (or $4.50 per day round trip), this is at least 10% of total trip fares.