From an engineering standpoint, consumers generally do not know the full extent of what things cost to produce. When I say produce, I mean manufacture for mass production. Tooling costs for molds is expensive, having employees to design the parts is expensive, basically everything about the process is expensive. I am sure there is a cost analysis that comes into play, at least it does at my company, where the company has a break even point and when they start making profit on the product, and I wouldn't be surprised if this was measured in years. The less demand for a product means an extended payoff of the development costs, or a higher priced product.
If you were to build a one-off kit for yourself, yes it would probably be cheaper in the long run, but you will suffer from tuning issues, the like that these companies spent a year or more tweaking, at their cost, to remedy. You have to remember these kits are developed for reliability and be able to have the results and the parts re-produced correctly every time the kit is installed or manufactured.
Like I always said, you have to have money to play this game, if you don't have the money, play a different game.