Have you ever done an electronics project? I have done many. Time. People value their time. Based on what I have seen and heard it isn't an easy project to build your own. That said, I do want to build one myself but been to busy.
I know that R&D is „expensive“ but in this case the reseller take a huge margin
You sound like a marxist. Not saying you are one but that's a classic misunderstanding of the price system. This is just the price system. You are aware of the markup on clothes right? I bought one from a seller that donates back to the project. I consider a portion of the cost supporting the project. By all means though, go build one. Or open a store and sell them at a lower price and corner the market..