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....
With mining devices, the revenue they generate over time must be properly understood when doing valuation calculations. You paid for your single earlier than most, but that doesn't cause it to be worth now more than other singles; you cashed out the benefit of your early investment with the mining output it generated over the time since receiving it. On the same note, if people are willing to pay $600 to get a single within 4 weeks, they'd be willing to pay much more to get a single now and have it generate income over these 4 weeks.
No it doesn't work that way.... pre-ordering something is usually done for the same price... even if you have to wait a long time... I paid exactly the same for my Black Ops II as someone who buys it in the store today.... Same goes for iPads, iPhones etc.... Sometimes you even pay more on pre-order than when it becomes readily available because of the exclusivity to have something first.
You can't deduct a higher value for a pre-ordered product just because you had a delay in delivery time. it doesn't make sense economically....
edit: Good luck Inaba on setting an exchange value that is agreeable for the majority