Just do it all yourself then? It doesn’t seem unreasonable that the creators of this want to make a little money off of something that almost certainly took a lot of their time. If the markup is too high you’re welcome to assemble your own.

R&D cost is built in to everything you buy, otherwise the companies that make this stuff wouldn’t be profitable. The amount of per unit R&D markup for, say, a thermometer just isn’t as high due to its relatively low complexity and huge number of units sold. This thing is niche and not going to sell that many units.