This is fairly normal for flights where there is no LCC non-stop competition. There are cheaper T fares available on this route, they just require a Saturday night stay (segregates the leisure flyers from the business flyers). US prices their mid-week non-stops in the same ballpark (~$1000). Since PHL is a US hub, you can play hidden city games (one-way outbound through PHL on US, and one-way return on DL through DTW), if you are really desperate to save money.