The Red Devils have been linked with a move to re-sign the Portuguese winger next summer, with several reports suggesting that Louis van Gaal is eager to add the Golden Ball winner to his squad.
Mendes, however, highlighted his client's price tag, telling AS: "In addition to being the best footballer in history, [Ronaldo] is the greatest athlete of all time. You cannot compare him with anyone.
"He is the best player in the biggest sport in the world, in which there is more competition than in any other. For that reason, he is the best sportsman in the world.
"The price that is stated in his release clause – €1bn [£800m]."
Ronaldo has scored 269 goals in 257 games for Los Merengues since his move from Old Trafford in 2009.