Hi,
There are two main mechanisms to limit relays bandwidth usage.
The first is by setting RelayBandwidthRate and RelayBandwidthBurst to limit the average and peak bandwidth usage of your bridge.
The second is by setting AccountingMax, AccountingStart and possibly accounting AccountingRule, to limit the total bandwidth usage over some periode (day, week or month)
If you want to limit your relay to, say 100mbit, with burst at 120, and set a hard limit on monthly traffic to 1TB you can set in your relay torrc.
RelayBandwidthRate 100MBits
RelayBandwidthBurst 120MBits
AccountingMax 1 TBytes
AccountingStart month 1 00:00 # this is actually the default value, so you can omit it
You can use either or both kind of settings. As the goal is to limit cost, you should definitely use the Accounting part. You can set RelayBandwdith so your relay does not use its quota in just a few days, unless that would make it too slow.