This has nothing to do with USD price of bitcoin...
I paid 300 BTC for them. Yes they were sold in USD but even if BTC were $1 each, I will most likely not be able to mine the 300BTC I paid for them back in a reasonable amount of time.
Of course not, but you will be able to mine back the cost of the unit in $USD in a reasonable time, and that's the point. The unit was sold to you priced in $USD not BTC, whatever you choose to sell to get sufficient $USD at the time of the order is your problem. Personally I would have used something that inflates over time, like a fiat currency to pay for an item with a long lead time.