If you think that designing and producing something that will reliably run, and hold up to 300 bar pressure, you are sorely mistaken. You are certainly welcome to build your own to prove your point. You could always go back to CO2, multi pumps, or springers.
i'm a new jack to PCP rifles and i'm hand pumping for now which really isn't too bad honestly. I do however think about a big bottle or compressor every now and then.....But like many, the price for what I believe to be a decent and reliable PCP compressor costs an arm and a leg. Why are these little rinky dink compressors so expensive? Some of these brands (won't mention names) look like a piston wrapped around some shiny sheet metal and are $500 + and most have very mixed reviews.What gives?
in addition to the above comments, I would also add: Even though it seems like there are a lot out there and available, it is about the quantity sold, too. If every house had one, the price would not be so high. (but then the consumer advocate groups would probably insist on a zillion safety widgets and the cost higher....sigh) It is difficult to engineer, manufacture and market any product for 'low cost' in low volume...and I suspect there aren't enough consumers available for this equipment to lower the costs much more than they are.
I paid $375 for my DeWalt that only goes up to 200PSI. My Nomad II that goes to 4500PSI doesn't seem so bad after that.