The DTW Metro Cab service would be an option if you just miss the terminal-to-terminal shuttle. But I don't imagine it will be particularly cheap. Note that McNamara is a single "terminal" (with multiple concourses). A flight that departs from the ends of A concourses won't be that much shorter to get to than ones from the middle of B/C concourses (via the tunnel). Particularly if you just miss the concourse A tram ride.