why not price it in USD or euro? bitcoin price fluctuating by several hundred dollars a day doesnt really lend itself to being used as a currency
but the bitcoins it mines becomes more or less valueable as exchange rate goes up or down. If you bought this for ~$1000 dollars and received it, then bitcoin tanked to $10 right after you received it, you'd be pretty pissed, wouldn't you? On the other hand, if bitcoin went to 5x the value after yo ureceived it, seller would have been screwed and lost a lot of money.
Bitcoin prices for devices that make bitcoins is probably best.