An interesting question. First, we need to estimate how dense a cloud is. We know that the cloud is floating (otherwise it will fall down!) so that is must be less dense than dry air. Lets assume the cloud is 1000m high, which is about the height of the tallest mountain in England. Now lets assume the cloud is about 100m thick, and 1000m long and 1000m wide (all reasonable). The weight of dry air is then (100x1000x1000) m^3*1.07 kg m^-3 = 107 million kilograms (!)
So the cloud weighs less than that. We know that a cloud is when the humidity reaches 100%, so we can estimate the density of completely wet air which is about 0.9975 times the density of dry air (we can check: this is less than 1 so the wet air is less dense than dry air, so the cloud floats, check!) that means we can estimate the cloud to have a mass of 106,700,000 kilograms.
Or, about 7000 double decker buses! Quite surprising, no! That isn’t a particularly large cloud either.
The thing about clouds is they are the stage just before the water falls out of the sky, so they contain all the water that comes down as rain, ust on the verge of being dense enough and heavy enough to fall out of the sky. So think about how much water comes down in a rainstorm, and that is pretty much your weight of cloud!