Hey Jens,
I just opened the virtual doors of my store this morning, So I am not rich yet 
Also, I don’t expect to get rich by selling Kirby plugins, the market is just to small. If you develop something like Advanced Custom Fields or the Enfold theme for WordPress, that might work. Although, I don’t think that most of those authors speculated on making a lot of profit from selling there products.
For WordPress, there is a whole ecosystem, that only works on a large scale. Take WPML (bring multi-language websites to WordPress … yes, that’s not built into core) for example: They sell you a plugin suite and you have to pay an additional $ 50 every year to get updates. That might work pretty well, because the scope of stuff that their plugins can hande is very large and there is also a large target-group (basically everybody who whants to build a multi-language website with WordPress). Also, using WPML generates a “nice” lock-in effect. Once you use it, you have to keep on using it. Like Advanced Custom Fields Pro, WPML let’s you use a single license for every website, you’re running. Because there are probably hundreds of thousands of customers at least, this business model should work just fine for them (at least, their companies still exist …).
But with Kirby Plugins, this is a little different. We have a smaller target-group, made up mostly of web professionals, building products for themselves or clients. One advantage of this: I believe people are willing to pay for quality plugins/themes, as they also pay for Kirby, especially when working on a large-scale client website with a budget of thousands of euros.
As ImageKit is a very handy tool for my own and my client projects, I thought about sharing it with the world. I did not decide to charge money for ImageKit because I’m greedy, but to help me keeping up with the development and documentation. I know that a lot potential users are going to make a living by making websites, so why shouldn’t they support me if my plugin makes their lives easier? Implementing your own solution or at least writing a proper thumb component that talks to a cloud-based image-processing service is probably more costly, than spending $ 25 for a plugin.
But for now, it is too early to answer your question. I’m excited, how many people are going to use it. Let’s talk about that again in a few months. 