Is there a way to limit bandwidth of a backup/restore from a network location?
If backups/restores are the only activities occurring between the SQL Server and your offsite Data Center, you could setup Firewall Quality of Service rules to limit transfer speeds.  The total amount of data you transfer can only be reduced by lowering the amount of data you send (such as only sending every other backup).

I will highly suggest that you adjust where your backups are created, though.  You should backup your databases locally (preferably to another server within your local data center or to a separate backup drive locally on the server itself if you don't have other options available), and then configure a job to copy the backups (or every other backup) off site for long-term disaster recovery purposes.  Not only could this help reduce your bandwidth issue, but more importantly, having at least one backup locally will eliminate the possibility of a network outage or some other external factor from preventing you from recovering the database in the event of an emergency.  Backing up a database directly to an external data center is a risky proposition as you can generally count on some sort of network issue every now and again.  I've seen far more network related outages than disk or server outages, though my experience may not be reflective of everyone's.