Compare park and ride with driving.
Park and ride replaces the expensive end of a commute — the congested miles and the city centre parking — with a fare. The comparison is therefore between full-distance fuel plus downtown parking on one side, and short-distance fuel plus lot fee and transit on the other. Downtown parking is usually the dominant term: at 22 dollars a day it is over 470 dollars a month, which a 5 dollar fare and a small lot fee cannot approach. Convert to a monthly figure using 21.7 working days rather than 20, and the saving becomes concrete. What the money does not capture is reliability — the transit leg has a timetable, and that cuts both ways.
Park and Ride Cost
Drive all the way = 2 x full miles / mpg x fuel price + downtown parking; park and ride = 2 x miles to lot / mpg x fuel price + lot fee and fare; monthly saving = daily saving x 21.7
Almost always where city centre parking is charged, because parking dominates the sum. It stops making sense if parking at your destination is free or if the lot is barely closer than the destination.
Park and ride typically adds 10-20 minutes each way in waiting and transfers, but often removes the least predictable part of the journey. Compare the variability, not just the average.