I understand your feeling!
Like @joumana says, however, it’s not a competition “who gets the most value for the least investment”. It’s about supporting YOU as an artist (or whatever you consider yourself to be - I don’t consider myself an artist at all, but do welcome the support I get nevertheless).
If your argument would hold, the price for each new revision of a product - a car, a pizza, a newly bred type of dog - would have to be potentiated by the number of revisions done. Imagine you buy the latest brand of a good car, by your own standard you would have to pay for the current model, the previous model and all the models before, because you get the improvements each older model showed PLUS the latest improvements. Nobody would buy a car on that premise.
Which might be a good thing btw. Excuse the digress.
You could claim that art is not revisions. But it is. An artist that does not get BETTER with every artwork she publishes, is boring and is not worth any support (because what for would the support be?), so each new artwork is an IMPROVEMENT.
If you want to get paid for a specific piece of art or a specific piece of work, apply the pledge to that specific item, not to YOU as the provider of an ongoing process of improvement. Your patrons, if you are getting some monthly income from them, are supporting YOU. They support you for the work you did so far, they support you for the work you will do in the future - not for an AMOUNT of work that they “buy out” from you.
Again: You can sell your work, one by one, if you desire so. But don’t penalize someone who found out about your work just now by (wanting to) make him pay for all the time he missed your work.
Marc