I like to think parity in terms of ~380 ounces per BTC. That's due to the ratio of 6.1 bn above ground ounces vs 16.15 mn bitcoins mined.
That would require a price of $456k per BTC - but at least the scarcity ratio would be accounted for...
As a side note, the problem with gold spiking upwards is its large marketcap and large annual production levels. At current prices, ~3500 tons of gold per year of new mining output (without factoring recycling) is ~110mn oz. That requires 132bn USD to absorb. A tenfold increase in the price of gold, would suddenly require 1.32 trillion USD per year just to buy annual production. The problem is that there is no such liquidity in the system for allowing this.
On the other hand, silver or bitcoin, can do much larger runs due to their smaller marketcap and much smaller liquidity requirements to buy their annual production.
BTC at 10500$ (10x) would require just 6.9 billion per year to buy the annual production (657k coins x 10.5k usd).
Silver at 170$ (10x) would require 136 billion per year to buy the annual mining output of ~800mn oz.
Gold is priced so high that the numbers involved are too high at 1.32 trillion USD (in a scenario of 10x price) for its 110mn oz. It could happen in a hyperinflation scenario, but then the money one takes wouldn't be worth it anyway. In a sense, gold is constrained from doing a huge run by its large marketcap and the liquidity requirements in the fiat system to sustain prices of 10x+. Silver less so, and bitcoin even less so.
Bitcoin seems to be the best bet in terms of upwards potential because it's so small and its fiat requirements to sustain its rise and mining output absorption are equally small.
Some interesting points in here, especially the the scarcity ratio
comparison between bitcoin:gold ...
... anyone watching this may also be interested in following this thread
Gold Price Parity Watch
... And probably the best answer will be : "fake funny paper monopoly money that people believed it was real and held the illusion of actually having value!" ... kids will scratch their heads a bit when hearing that... 
