I find this strange that DTW-SFO is more expensive than GRR-SFO. Both the DTW-SFO and DTW-LAX and return are usually cheaper due to the competition with Spirit. If the flight time is not in competition with the Spirit schedule, then it might be different.
I have flown on what I consider low fare for DTW-SFO leaving at 3:39 pm and returning on the red-eye at 10 pm.

HJ