I have flown the DTW-PHL-DTW route quite a bit for business and have rarely had it price less than $800-$900. Also, though there may appear to be open seats that far out, close in they will fill up and it would be unusually if more than a handful of seats go unsold. Upgrades are also tough on these flights as well. I sat next to a DM last Friday on the way back to DTW that was #7 for 4 and he said he also missed the upgrade on the second flight out to PHL on Monday morning. I was on the early flight to PHL on Monday morning and as a PM, I ended up #8 for 0 on an M fare.

Good Luck !