The price of a seat has next to nothing to do with the actual operating costs for that flight. It is all marketing and competition.

If you want to get from GRR to SFO, you probably have zero options for a direct flight. That means lots of competition since pretty much any carrier that serves GRR can get you there with one stop. From DTW, your only direct option to SFO looks like NW and one Spirit flight. Less competition for a better product (non-stop) = higher price.