I think it really depends, Beirut - mostly on what bitrate those MP3's are encoded at.
Just by eyeballing some samples (and I'm not even sure the same ripper, with the same parameters, was used to create all of them), 320kbps is gonna be more than double (in size) compared to 128kbps.
If you have variable rate MP3's (VBR), which aren't that uncommon, it gets even more complicated. So it's hard to estimate how much space it would take beforehand.