Localbitcoins history btc_in_usd rate?
It's possible to get your price equation via their API:
https://localbitcoins.com/api-docs/#ads
As well as the price at which each of the trades are locked in at:
https://localbitcoins.com/api-docs/#dashboard
It should be possible to write a script to scrape this data and calculate the btc_in_usd value using this.
(I'm sure this information is available somewhere in their user panel too, although you might have to look around)