Downtown rents in Detroit were very, very high compared to the surrounding area because of workers desiring to live in a very small radius of the immediate downtown area
primarily
to be close to work and downtown amenities/restaurants.
QuickenLoans would be the biggest employer downtown right now, not the car companies, and other tech companies have been moving in as well. Rents were exacerbated by a shortage of apartment stock, homogeneous ownership of apartments (i.e. by Dan Gilbert), and lack of affordable parking in a city where cars are still required. Rents outside the downtown area can be almost half as much because of this, despite all still within a 10-20 minute drive.
Overall, I've seen most downtown rents drop by several hundred dollars, usually by maybe a fifth of the previous amount (which seems very close to the percentage mentioned). Outside of downtown, the drops have been MUCH less steep, if at all.