This information applies to Terragen 4.0 to 4.4, but might change in future versions.
Each voxel may take up between 12 and 24 bytes, depending on whether it's visible in the image and whether it affects the lighting of other parts of the scene. In practice you will often find that, for large cloud layers where only parts of them are visible on screen, the memory use will be closer to 12 bytes per voxel. If most of the cloud is visible in the image you can expect it to get closer to the 24 bytes per voxel limit.
There is an additional buffer for cloud GI which is much smaller than the above, but has the potential to push the RAM use above 24 bytes per voxel.
In the 3D Preview the RAM use is much lower but also more variable (and likely to change between versions of TG), so it's probably not useful to try to use that to estimate the RAM use in a full render.
The clouds might need additional RAM to calculate GI during rendering (e.g. using more subdiv cache), but if you're rendering a typical scene that already uses the subdiv cache it probably won't make much difference.
I've done some tests to see if anything I've said here is wrong, and so far it seems to be about right, but there may be other factors which I've forgotten to account for.