A back of the envelope calculation based on my CDs-ripped-as-FLAC collection yields an average of 331MB per disc. That's mostly albums (the few singles and compilations cancel out, size-wise). A CD does indeed store about 700MB of PCM audio, but FLAC compresses that to a little over half the size, and most discs[1] don't use all the available capacity. (Obviously you can achieve a much higher compression ratio using lossy codecs.)
To muddy the waters, computers - operating in binary - calculate file and memory sizes in powers of 2. So 1TiB = 1024GiB = 1048576MiB. Disk[1] and network equipment manufacturers, on the other hand, tend to use decimal SI prefixes, so 1TB = 1000GB = 1000000MB. This makes some sense for telecoms, but is mostly about making the numbers sound slightly more impressive when it comes to storage. (The IEC coined the infrequently used terms 'tebibyte' 'gibibyte' 'mebibyte' for the binary prefixes in a mostly-futile attempt at clarification.)
In the real world, we tend to handwave over the difference, and just bear in mind that a given volume will have a few percent less usable capacity after the filesystem overheads and conversion to binary prefixes.
My rule of thumb for buying storage for domestic use is to work out how much you're probably going to need in 2-3 years time (the expected lifetime of hard disks), and buy whichever size has the best storage:price ratio above that.
[1] By convention, optical discs are spelt with a 'c' and magnetic disks are spelt with a 'k'.