TH, announce the date and time when you open up orders very clearly and at least a few days ahead. That way most if not all people get the same chance.

On subject of pricing things in BTC, the thing is that you are looking at it backwards. It's not the machine that gets cheaper or more expensive as the price moves around, it's that the worth of your bitcoins purchased at a given point changes. If you want to play around with that, what you do is buy BTC and then later sell it if and when it goes up, and then you'll have more money - doing this will yield the exact same results as them pricing things in BTC. That being said I'm neither for or against the BTC-based pricing, it could easily backfire on the customers but it's impossible to predict anything.