And if yes what should I say?

Is the brides parents paying and planning the wedding ?

It is proper (and good common decency) for anyone who benefits from something to thank those who paid for it.
In the case of a wedding, it is more a situation of letting them know how beautiful and thoughtfully planned it was, and how much you appreciate all the work and time and creativity, rather than mentioning cost directly.
And you are so happy that the woman your son loves has parents as wonderful as she is, as you knew they would be, and so on. Lots of warm feeling and some hugs without being fake about it.

Now we hear on this site about weddings in some cultures where one family literally demands money from the other family right after the ceremony and just before the festivities begin. I can't answer to that. In many parts of the world, the bride's family pays for almost everything, with some exceptions like a certain event like a 'rehearsal dinner' or something. It's all known in advance.