You should calculate everything and move your prices up as you have unlimited bandwidth. I have few static websites, but smallest one generates on avg. 400gb monthly transfer(plain website with main content), adding to this image optimisation(sizes, etc) and few other options totalling nearly 1TB+ a month.
So for me $15 looks nice, but you would loose a huge amount of money on clients like me. You can't expect that static website will weight 1MB and eat 1GB of bandwidth a month. :)
This is why Cloudflare has image optimisation in Business plan, or Imgix which is pricey. :)