Interestingly, some really good quality right wingers have gone for reasonable fees. Malcolm is an exceptional talent, and went for 40 million Euros.
The issue with wingers and United is that Mourinho likes wingers who track back like crazy - Willian, Perisic and the likes. Fair enough, the guy has his style - but I'm pretty sure you can get a good run out of Martial/Sanchez on the right, stick the other on the left, and you have a 25 goal a season striker.
At the same time, if he adjusted his playing style a bit, a manager with Martial, Rashford, Lukaku, Sanchez, Mata and Lingard shouldn't really have issues with getting his team to play good attacking football. 2 of them are world class (close to it in Lukaku's case), then there's 2 outstanding talents with pace to burn along with reliable and versatile options in Mata and Lingard who are both productive goals/assists wise.
Yes, a speciaillist winger would be good, but its not like the squad is lacking good personnel in attack. Assuming Lingard starts at RW, your backup front 3 is Martial, Rashford, Mata. That's better than every team in the division barring City!