Well, it is sort of a "You get what you pay for" situation.
One reason is they have to pass rigid testing and approval, because they are life protecting devices in industry. Industrial products are always higher priced than hobby use devices.
Two, the batteries are usually very model specific, which always creates a high price for the battery.
Three, most PAPR systems are made from much better grade materials, as they are used every day in industry.
Shopping around on Ebay and other sites can find good PAPR systems for a fraction of the new price. Sometimes, the best deal comes from buying the hood, blower, and batteries separately.
I use a modified Breathe Easy system.