It depends a lot of nature of business.
For resource intensive business (e.g. video processing, big data as a service)I would expect 20-40% of revenue, so 2.4 - 4.8 mln/year.
For typical application (e.g. CRM, specialized app, integration) 5-10% of revenue, so 600k-1.2 mln/year.
Mature, stable companies tend to have lower numbers. Hypergrowth startups may have waste in their growth phase.
Happy to help with your AWS optimizations.