I know even newspapers do this kind of compairment, but this tells nothing.
One way which might be a reasonable comparison would be comparing the price of the total amount of gold mined till now, to the price of the total amount of bitcoin mined till now.
I guess we'd have to factor time into it, we will have to compare them from the time of Bitcoin's inception right until now and that will make sense to draw the correlation between the two in terms of prices.
Why and how would you factor time into it?
Of course you have to compare them over time, like two graphs with one X-Axis (time) but factor it in?