Bayern are keen to sign the 23-year-old Spain international this summer, but need to pay the full buy-out clause for the midfielder to do so.
"I hope that he will do really well at Bayern, becomes one of the stars of the Bundesliga, and helps them to the league title," Effenberg told TZ. "There's a big risk, though.
"I have been asking myself whether he's worth this kind of money, and I don't think that's the case. It has surprised me a bit that Bayern are so desperate to get him. He is not even a regular starter for Spain.
"Yes, he has won the World Cup and the Euros, but he played only 25 minutes at Euro 2012."
"It's clear that he's not on par with the big guns like Iniesta and Xavi," said the German. "That's why I'm wondering whether he's worth the money."
Martinez has been left out of Bilbao's squad to face HJK Helsinki in the first leg of their Europa League playoff tie on Thursday.