I mean I would be limiting it to a very specific use case, and the flat fee would be "plus a percentage of the underlying", something like $50/mo + 20% of the AWS spend underlying.

And for that, with a simplified use case, well, they can scale up to $20k+ a month if they like, that would be ideal, and if they need a more complicated setup or enterpriseyness, migrate off with my blessing and available-not-required hands on support (again for a reasonable fee, maybe $10k if you want the white glove).