You can of course get 200 gramme thinsulates too... if you are one of those lucky people who live where the water is warmer
*hint* fill out your profiles so that people know where you are based then advice can be location specific*
Thinsulate used by DUI (and everyone else as I understand it) is made by 3M. The type B 400gm resists compression (B stands for "Boot" and thus has been designed for this purpose) The dense fibre also resists saturation if flooded - absorbing less than 1 per cent water/weight.
If an undersuit is expensive - it is probably Type B... or DUI... or both :D