Acoustic underlay (underlay arriba arriba)

[drive2tractors]

Has anyone ever fitted sound deadening underlay? Was it expensive and are you happy with it's performance. At the end of our restoration we are considering relaying the carpet over this material to try and insulate sound from both floors. It seems expensive and would appreciate your comments as to it's value.
Thanks
d2t
Ps sorry for the thread title hijack.