It's difficult to be more specific without knowing your travel dates, but the current cheapest published fares from DTW to MAD are:

CO: $422.00
NW: $514.00
DL: $532.00

Of course, the price you end up paying will be more once the taxes and international fees are added, but this will be the same for each carrier.

As you can see, each carrier has a different price. Is it possible that the cheaper seats on a particular flight you were looking at happened to sell out, bumping that flight to a higher fare?

It's funny when it comes to the pricing of codeshare flights: no matter what the situation, a large group of people is screaming about it being inequitable. If the prices are the same, then people complain about price collusion, even though prices are often identical on non-codeshare routes (hint: airlines can see each other's published fares in the CRS's without using collusion). If they are different on codeshares, then another group of people pipe up and point to this being yet another example of the screwy airline pricing model - after all, how can prices be different from two carriers that are selling seats on the same plane?

On DTW-MAD, both AA and CO have $422 fares right now. I hardly think this is too expensive for a trans-oceanic flight. They are about $100 cheaper than the cheapest NW flight, but that's common pricing for non-dominant carriers in a particular route or market (it's called competition).