If you pre-order with one Bitcoin today, then you paid the dollar value of one Bitcoin today, plus transaction costs. Future fluctuations in Bitcoin value are irrelevant.
Theoretically, you have to use the real rate of return of your money and discount it on the time period. Of course, estimating the real rate of return is an art in itself. Considering value of bitcoin has been shown to be highly volatile, doing any semi-long term investment with it is basically gambling.
With a pre-order on the other hand, you give them the money and you get nothing for four months. Probably longer.
If the value of Bitcoin fluctuates between now and the time you receive your product, then you may "lose" or "win", but that's a chance you take, even with the product in hand. In other words, there is still the possibility that your 1 Bitcoin could have allowed you to purchase something more valuable than the product in hand at some point down the road.
Seems that the only thing you give up with a preorder is the interest potential or use of the money for some period before receiving the value (i.e. product). But, that would be the case with a preorder irrespective of the currency used.