The GoingPrice_US addon uses the LibExtraTip library to display various statistics from us.battle.net. The statistics are sampled over all realms in the US region. To get realm specific data, head over to http://goingpriceaddon.com/download to download realm specific addon's - as many as you want. I suggest all the realms you have characters on. Install them together with GoingPrice_US. The GoingPrice suite is smart enough to load only what's needed.
These addon's should update quite frequently - about every hour or so for realm specific addon's and daily for the region addon.
If you find any of the GoingPrice addon's useful, please consider making a donation.
Release versions take the form of 5.2.xxxxxxxxxx where the '5.2' part is the version of the Wow interface supported and the 'xxxxxxxxxx' part is the date (in seconds from epoch) that the latest price was pulled from us.battle.net