It's a variety of factors that make up the price of a rebreather. It's quite easy, and fairly inexpensive, to build one. Parts were pointed out in the first post, and if you search this forum or the web you'll find links to the "$100 rebrather", and, if I recall correctly, even the "$1 rebreather". However, it's quite a different matter to build a good rebreather. That takes quite some craftsman skills and engineering knowledge. Unless you've got it all and do it for yourself, neither the skills nor the knowledge comes cheap.
Next comes the testing, ranging from a homebuilder hitting the local pond to preparing production, paying for actual testing and in a couple of cases CE certification (that alone is upward of 50K, more than $75K!) ... if all the homework was done in the first place and no changes are needed to pass.
While you ramp up production, you need to get an instructor base set up, too. Usually that starts with the boss and a couple of more or less experienced key instructors diving the prototypes and early production units. Ain't nothing for free, they either keep the rigs or get them for very little.
Next, so you can sell that "really expensive" rebreather you need to actually build it.
Broad variances here, too. Some units are made mostly from molds, such as the Inspo/Evo, Sport Kiss and the Optima. That translates into very high tooling costs, but allows for relativly cheap actual production runs for those parts. Others, like the Classic Kiss, Meg or Boris are almost entirely machined. Makes it easier to change stuff, has considerably less tooling costs, but is only price effective for low production numbers. Both skilled labor and less effective use of raw materials make machining an expensive proposition.
Last but not least, everyone involved in the project needs to make a living. From whoever keeps the books and answers the phone to the guy packing and shipping the rigs. And they should. Having had the opportunity to meet most of the manufacturers, and visit several of them, I can tell you that none have made riches so far from being in the rebreather manufacturing business. Some make a decent living, others don't really. Some have other sources of income, manufacturing, engineering and/or selling other stuff.
As for the costs mentioned in the initial post, civilian RBs are somewhere between $5K and around $18K, with the majority costing around between $8-9K. All depends on the model, the options and the exchange rates in case of foreign products (which currently really sucks for US buyers). The $50K mentioned are for a military unit where a single tank made from non-magnetic inconel costs over $1K, the piezo-electronic O2 injection valve costs almost as much as the average civillian CCR and whose target market has very deep pockets.
For used units the market obviously makes price based on availabilty and demand. Used CCRs can be had for $3K or $15K.
As far as product liability insurance is concerned, I'm not even sure all manufacturers can afford it. Especially in the US I would be surprised as such insurance coverage is outrageously high.