You will probably need to look at how you're currently performing backup and see if there opportunities to optimize it. You could potentially benefit from link aggregation as your source data is coming from multiple esxi hypervisors.
As for the 10Gbe, this is a business decision, I don't think looking just at cost is a good thing. Backups are insurance, if you need to have a full backup within appropriate backup window, then you will have to build a solution that provides.