To build a comparison of price indexes you have to have an exchange rate and products that are sold on two markets that use the two currencies separately, there's no Big Mac sold by BTC as far as I know, as such you have to find products that are sold in the market using the two currencies.

I can think of pot, and using silk road to build the index for BTC but as I see its prices are pegged to dollar and BTC is only used to maintain some anonymity of the transactions.

I'm not sure that if that's what you wanted.