How would you define "dirt cheap"? One of the most popular CDN's out there (Akamai) charges $3,500/month for 10TB/month. Who's going to foot that bill? :)
That's a list price that they charge those who don't care to negotiate.
CloudFlare starts at $0 and doesn't meter/charge for bandwidth. CloudFront charges 9 cents per GB and is integrated with other AWS APIs (which can be very useful). Both those solutions could be managed with a donation pool, I would try the CloudFlare free tier first.
Not surprising, Akamai is always at the top end of the pricing spectrum for CDN services.
That price point works out to $0.35/Gbyte. More typical list pricing for US/EU is in the $0.10-0.15/Gbyte ballpark. Prices decline rapidly as your utilization approaches 1PB per month.
I believe that's off by an order of magnitude - that's actually the price per 1 TB. Also, you can turn off distribution into remote PoPs that charge more than $85/TB.
