As an ex tank commander in the Royal Armoured Corps i can vouch for the standard green army bivvy bag. I used to sleep on the back decks of a tank in all weathers and it would keep me dry. However be aware that although the bags are Goretex 'breathable' they are not human lungs breathable
The one time i slept with my head inside the bag with the drawcord done up tight....i woke up panicking, not able to breathe properly, i ended up sucking air in through the sphincter like draw cord hole. Other than that they are pretty much a solid bit of kit. I think i paid £18 for mine, though it was part of a large buy. There is a company out there (can't remember the name) who sell batches of ex mil kit with a minimum order of £200.00. Sorry i can't be more helpful.