Correlating BTC price changes with recent use in commerce
That's the thing about a pseudonymous digital currency -- there's little information that can be gleamed from the blockchain data.

There are some metrics reported in aggregate by the payment processors and others.  For instance, last month BitPay shared during a discussion that they had surpassed 10,000 transactions since their launch over a year earlier.  They didn't provide the number of bitcoins as well or give the average amount per-transaction.  They are probably the largest payment processor but even that amount is pretty insignificant.

This data will probably be most accurately obtained through user surveys and from there extrapolate.