A problem with this that I see (as a BTC->silver person) is that it doesn't, and can't really, take into account the BTC->$currency ratio. By which I mean, today, the low in BTC->USD is 1BTC->2USD, and that's not within what I'm willing to trade on.
Well, I had customers from where the BTC was 10$ and they said "the BTC will rise, and gold will fall".
The other way around! What I mean: You never know. Feel free to hold on to your BTC and come by my shop at 20$ :-D
I understand your concern. However, noone that buys metals can predict the USD/Metals ratio^^
Really, if you have a reasonable amount of BTC to save up from and do so over a period of time, the average price effect SHOULD work out in your favor.