Project Wonderful price index

If Project Wonderful is like the stock market, then it ought to have indexes, right?  Even if it isn't, it seems like an index of the overall level of prices on Project Wonderful would be useful.  I've observed that the bid prices on my own ad boxes seem to all go up and down more or less in synch with each other, and also in synch with the prices on others' boxes.  There are also theoretical reasons to expect that.  It seems like a lot of the money being bid on ad boxes in general is recycled ad revenue from other boxes, so when some boxes go up there's more money to spend on others.  There's also stuff going on like campaign bids (which make up almost all the incoming bids I see) all resetting at midnight and then running out of funds over the course of the day, so we'd expect overall prices to rise and fall on a daily cycle.  How do we know the current status of overall prices?  How do we verify things like what I said a year ago about large sites being more expensive than small sites? [2233 words...]