I started out with the Van Compass Stage 3 and am now running the Stage 6 kit. I have had 265/70/17 KO2's mounted on Method wheels since day one. I am contemplating moving up to 285's, but at 35k the KO2's still have quite a bit of life left in them. I am very happy with the Stage 6 kit.
Agile's kit will raise the van 1 to 2 inches and (because of the coil over spring) requires wheel spacers. Also, John recommends no more than 275's. With Van Compass you can do the suspension mods with or without the lift and spacers don't become a consideration until you start thinking in terms of 295's or larger. If you haven't seen it, look to this link on the Van Compass site for tire size information with their various configurations: https://vancompass.com/pages/frequently-asked-questions
As I said previously, both kits are represented here on the forum, and owners of each are happy. I am aware of another kit on the market, simply put, I would give it no consideration. Van Compass and Agile both provide quality, they just go about it differently. The two posts I linked above provide plenty of information and opinions, hopefully you will find enough there to help you to understand the differences in order to allow you to make an informed decision that best fits your use case.
As to your original question of why are the kits so expensive? Look at what you are getting, think of the development time and energy it took to get there, and know that with either choice you are going to be dealing with reputable vendors who will be there when you need them. I consider it money well spent.