I fetch the dumps twice per hour (for redundancy in the face of occasional downtime, late ticks, other unexpected stuff). By the end of the round (when the dumps are much bigger) it totals around 330Mb/month (based on rd10.5). Signups are up this round, and there's a new race column, so I'd expect it to be more. That can be up to a couple dollars of bandwidth charges depending on your host.
That said... I'd be pissed if it suddenly started being zipped cos I'd have to waste my time digging through dusty scripts and change things to unzip em, like I did last time they changed for no reason
[Edit: Ta for finally adding alliance and races dumps btw, very nice
]