You're probably looking for something along the lines of mod_cband, calculate based on 200GB per Mbps at 95% and allocate accordingly -- it'll allow clients to burst for small-ish files but fairly effectively ratelimits against 'big' transfers, and can enforce a global cap on transfer per day, week, month :)
I failed to consider that you may be using FTP and/or rsync too ;) Implement something with `tc` to QoS the traffic to a certain rate, and that'll also allow you to handle small/short bursts.
Long-term I consider "Mirror is a little slow for big transfers" better than "Mirror only works for 10 days of each month, then returns errors."