How much "protection does a building offer"

[DRG]

In a general "rule of thumb" way, yes but how well a unit does under fire and the casualties it may take are more related to that units experience, morale, suppression and status, and the firing units experience, morale, suppression, and status. The cover it might be in is another factor in the equation

An experienced unit has less chance of taking casualties than a less experienced one generally but the game deliberately has multiple variables thrown it to keep things "interesting" so sometimes a unit can "get lucky" with its cover and concealment....and sometimes not