By incentivising good design and efficient code, would it be possible to run a web business on low enough margins that you could make profits just from the initial signup fee?

Considering some courses I’ve bought, the answer is yes. Tailwind UI is another example. However, to take it a step further, would it be possible without charging the exorbitant rates of £3-400?

It’s not like you have to stop innovating. DLC / “expansion pack” model probably work. You just have to ensure that bloat (read: AWS spend) is avoided as a core principle of your work.