Nani has attracted interest from Manchester United and Tottenham after impressing this season, but the Portuguese club are adamant that he will not be leaving cheaply.
It is believed that £13.5 million would be enough to land the player, though, as he has a clause in his contract stating offers of that amount have to be accepted.
A Sporting spokesman told skysports.com: "The club that wants the kid have a price, £13.5 million. Only by this money would we be disposed to let Nani leave.
"We know of the interest of several English clubs for Nani, and that is normal because of his quality - but until this moment we have received no offer in writing."
Ronaldo joined Manchester United from the Lisbon club in 2004 and has gone on to establish himself as one of the best players in the World.