You can cap the inbound / outbound speed.
Right click on the Edge Gateway provided to the Organization VDC and go to properties.
There should be a tab visible labeled "Configure Rate Limits"
From here you can enable this and choose a maximum upload / download speed.
I've not seen anyway to limit the actual monthly usage though as far as saying they can use a total of 200GB per month of bandwidth, pretty sure this is not an option.