I think you'd be better served to ask about the average volume of a cloud (if that makes sense, given how diverse they come). Because the mass is pretty much exactly volume multiplied with density. And the average density of clouds is pretty much exactly that of the surrounding air at the given altitude (because otherwise the cloud would not float, but either rise up or sink down). And the density of air at any given altitude is given by the Barometric formula. If you take a kubik kilometer of cloud (honestly, I have no idea how big clouds are), it would have a mass of approximately 364 thousand tons at 11 km above sea level, 88 thousand tons at 20 kilometers above sea level, 860 tons at 51 km above sea level and ca. 64 tons at 71km above sea level. But "regular" clouds only go up to ca. 20km, and 95% of the clouds you see are probably below 8km. Unfortunately the quoted wikipedia page has no entry for the barometric pressure at that altitude and I am too lazy to try and calculate it right now ;)