The mistake here is assuming that a mini-rig is worth $15295.
At the time when the devices were bought, $15295 didn't get you a mini-rig. It got you a mini-rig to be delivered several months in the future. An actual mini-rig in your possession is hence worth much more than $15295.
At the same time $600 would get you a single within a few weeks. So a single is worth more than $600, but not by much.
So my Single ordered in November 2011 and delivered in April 2012 is worth more than a single ordered in July 2012 and delivered in September 2012, just because I had wait longer to get it.... This means that Inaba has to take into account the delivery time on the singles and then set a different price for every single according to the delivery time it took to get them....
Somehow you can see that your argument is not quite right....
BTW same can be said for anything you pre-order.... my Call of Duty Black Ops II which I pre-ordered is not worth anything more than the ones people can buy directly from the store