"Les athlètes professionnels gagnent beaucoup d'argent."
Translation:Professional athletes make a lot of money.
"Much" works just as well as "a lot of"; indeed, in carefully written prose, it is a better choice. Yes, one hears a lot of "a lot of," but give a little credit to those of us who remember the old but not superannuated handbooks of usage.
It can work with "much" but it does not sound like natural English. This is a case where "a lot of" sounds more natural. If it is too informal for you, you can also use "a great deal of".