>It would be one thing if the app relied on infrastructure
Look into value based pricing. It's a mistake to charge based on how much something costs you to produce.
As an example, if I make an SEO tweak that makes a company an extra $10M in profit a year, should I charge for the hour of effort it took? Or should I charge differently?
Why is it a mistake? I mean, I know value based pricing is _possible_, but you're saying it's the only way?
Well to continue the example above, invoicing for a million per hour (e.g. 10% of the increased profit of 10M) is possible but definitely a bit strange. You probably want to structure it differently at that point.
Or to give another example, if it takes you 1 minute to adjust something which boosts someone's profits by 20% permanently, should you invoice them for 0.02 * hourly_rate?