risk vs reward here...
If you get to the 4th round and all of your guys are off the board, maybe the risk becomes worth it. Or if you have done your board homework and you don't feel the value is going to be there, you make the trade before the draft. The Cowboys have a good relationship with Chris Peterson so if he vouches for the former Husky, that would carry some weight with the team.
Some things to consider: In trading for Ross, you get him for a 2019 cap hit of $2,020,988 and a 2020 cap hit of $2,798,982. Since he's a player received via trade, you don't inherit the prorated bonus from his signing bonus. If he plays well, you can also trigger his 5th year option for 2021. The Cowboys just saved $2.25M by moving on from Terrance Williams. They could save another $4M if they get out of their feelings and make the sound decision to not pick up Allen Hurns option. They also have Cole Beasley and Tavon Austin hitting the open market in 2 weeks. Cooper's ability to play inside means you can add a 3rd receiver who plays outside, as opposed to a true slot guy. Ross also can return kicks so he adds value there.
To me, this comes down to two things:
1) Do they value Ross based on their own scouting and coachability concerns (any issues in Cincinnati, Peterson's take on him)
2) What do the Bengals want for him