The forward, who thrived under Villas-Boas at Tottenham Hotspur, is expected to leave the Santiago Bernabeu this summer.
"I don't know what market Madrid can operate in," Villas-Boas said.
"There are good players out there, but there are few like Bale, that's for sure.
"He's a top player, but Madrid have the economic power to pull someone in - whoever in the world they want."
He added: "For me [at Spurs], he was a fundamental player and made a difference whenever we gave him a little more freedom.
"His CV speaks for itself, and he has impacted big finals [such as the Champions League and Copa del Rey]. I don't know what will happen."