Yup, it's an expensive game. However if your mids are installed properly, and you can seal the doors properly. (needs fibreglass moulds on the clio due to the pocket in the door card), then it's defintatly worth while. Gains to be had on the midbass was definaty worth the ~£90 i had paid for my boot and 2 coats on my door cards.
Go onto TA and go onto SPL & streetbass section and flick through until you see sound deadening. The guy who runs DB drag, the most world known SPL format, is selling some for very cheap (what i used), well relativly cheap.
I still use pillows, in the hatch part of the boot, i did outer skin and inner skin the filled the middle with tesco value pillows. This has greatly reduced the hollows noise of the boot when it's rattling away with the torture of the bass.
Sound deadening is very complex and deadening refers to dampening a material (known as mass loading) it will only increase midbass really, as it stops panels rattling by lowering their resonant frequency.
For some better idea's on the fuller picture check my post in this thread.
http://www.cliosport.net/forum/showpost.php?p=5557445&postcount=5