The method you've used is similar to ones that FOREX traders use to value traditional currencies, which would speak to its validity. A market-basket of goods is defined, and a relative exchange rate for two currencies is based on the price differences of the two market-baskets.
A potential issue with this analysis is that your market-basket only contains illegal items -- however, even if your final valuation isn't accurate, it suggests to me that there are likely other cross-market pricing inconsistencies. I assume those are what you're interested in finding.
Pretty much, I'm simply trying to understand what the fundamental value of the currency is. The reason i'm using illegal goods is that they are the only ones i can find whose exchange must be in bitcoing (yes there are some other smaller goods but nothing on that scale).
A potential issue with this analysis is that your market-basket only contains illegal items -- however, even if your final valuation isn't accurate, it suggests to me that there are likely other cross-market pricing inconsistencies. I assume those are what you're interested in finding.