I have no experience with these, but
cdn77 charges $49/TB for US/EU location (not per month, you can use it until you consume I think)
So, per GB cost comes to 0.05/GB is
probably the cheapest I think
But anyway CDN is more costlier (5 to 10 times?) than normal vps/dedi I think (assuming vps providers don't oversell bandwidth like done in shared hosting). If you could have multiple (3) VPSes and do load balance with DNS, that would be cheaper than having a CDN I think (unless you want absolute low latency with 100s of servers across the globe).
The low end vps providers bandwidth cost varies from 0.005/GB to 0.01/GB.
Some unmetered ones might even workout to 0.001/GB (like ovh/etc) but not sure how reliable it would be.
cloudvps claims to offer a free global CDN, check for yourself.