There are several threads on TMC and videos on YouTube dealing with drag effect.
Typically you can expect 20 to 40% of losses.
In the case of roof rack or trailor hitch with bicycles, the measurements were done at 65 to 70 miles per hour.
But in the case of a trailer, the speed was lower, like 50 to 55 miles per hour.
In the case of a trailer, most of the comments were based on the Model X, but results should be very similar for the Model Y.