maybe he's the type of person that only downloads a small amount each month, like 5 gb, whihc would obviously be cheaper for him
True, but he did say the magic word, 'downloading'...
I have (still good) block accounts from over 10 years ago when I used to chase 'fills' (the servers/usenet system were nowhere as good as today).
But you're right, it all depends on the amount of traffic. But as the line/ISP costs are a pretty set number for the lowest speed (even if they are getting that as a bundle), the server costs (added on) above that are a very small amount.
Unless we're talking dial-up. But the cheapest dial-up is right around $10/month v. the $15/month for the cheapest DSL (or bundle from a cableco).
The cheapest total for both line/server costs is about $25/month, any lower than that is pretty much a 'forget all of it' proposition.