From 1-100% You could find any figure as it's highly dependent on the circumstances. I wouldn't want Paradox to trust what's written in the forum in any case when it comes to hard-facts as they're pretty slick in doing their own research (as well as having their own experiences). There are researchers out there that does a mighty fine job of actually doing their job so that us ordinary folk don't have to dig through the archives and read every actual war-diaries of divisions and doctors.
If memory serves me right close to 60% of the German casualties in December of 41' was due to weather/exposure (although most were operational-losses rather than "dead") and about 2/3rds of that number were from units that was mobile up to the Russian counter-offensive. Now I'm not stating that ALL of those units were not/less dug-in than on other fronts and the non-combat attrition-rates are also naturally higher in units in frequent combat due to a lack of hygiene, food, rest etc but the numbers are still high enough that one cannot readily disregard them as just a fluke.
I would recommend reading Operation Barbarossa: the Complete Organisational and Statistical Analysis by Nigel Askey for a thorough breakdown on German casualty-rates but I'm not going to start digging through my library just to bring you quotes. You can believe the statement or disregard it as you please.
It's not only about temperature but moisture, air-pressure, temperature, equipment, training, logistics/food/hygiene, lodging, officers, terrain, enemy actions etc etc etc and it's not a battlefield simulation we're talking about but a strategy-game. It might not even make sense to use historical/accurate figures due to game-play reasons and in any case a game on the operational/strategic level don't need to delve into casualty modifiers on that level of depth. I'm not trying to tell PDS HOW to develop a game (since they're clearly better at it than me) but there's not a word about digging-in/entrenchment in the new terrain-modifiers system and that is the reason I asked.