Madrid paid the reported sum as transfer fee to Spurs to sign the Wales international on a six-year contract, making the winger the most expensive player in football history.
"€100m (£85m) is a bit much," Van der Vaart told NOS. "You should ask yourself if a player is worth that amount at all.
"I've played with him, but I couldn't see that he was worth 100m back then. He did develop quite good, but still he has to prove his worth at Real.
"I know Bale as a very nice and sweet guy so I think he would fit best in England. A club like Real Madrid is different and there is a lot of pressure over there. But maybe he can surprise everybody at Real."
Van der Vaart joined Tottenham from Madrid in 2010 and left for Hamburg in the summer of 2012.