Correction - from my billing page it's $4.50 a month, from the resize page it is $6 so I'm guessing I am grandfathered in to some older pricing
Imagine the petabytes of data transferred through the internet saved if a couple RSS clients added that method.
If OP reduced 30 months of posts in rss to 12 months then this 13mb would be 5mb a day.
Using Cloudflare free plan and this static content is cached without any problem.
I think making clients behave correctly is much more sustainable solution, although we could do better than doing so at the cost of the end users.