Each tier, the formula slides down so that a normal-mode raiding chest costs 1000 GP. This means old gear prices drop. If you didn't update your EPGP, you would have been charging old prices (which would be higher than what an updated addon would charge).
The correct price for ilvl 522 bracers is 500gp. Thunderforged normal would be 587, Heroic would be 707, and heroic Thunderforged would be 830.
If you didn't update for MoP then you would indeed have charged 1000gp for bracers.
There are a few things you can do to correct this. If you open the EPGP options (hit escape, click Interface, click Addons, click EPGP) you have some buttons. One of them is "Rescale GP". This will reduce everyone's GP by almost half. The button was meant for Firelands when a tier was 19 ilvls instead of 26, so it won't cut everyone exactly in half, but it will be pretty close.
However, I don't like that; 19 is the wrong number. So here's a new release of epgp that uses 26 and thereby will cut GP exactly in half.
If you choose to rescale, please use 5.9.11 for it -- nothing before.
Now there is one downside; old GP, which is mostly-but-not-quite decayed, will also get scaled. This won't be much though at this point since 5.2 isn't brand new.
My recommendation, therefore, is to get 5.9.11, make sure everyone is using it, make a backup of your EPGP data via exporting from the Log frame, do the Rescale GP, and make sure things look like you want them. This should make things pretty consistent and let you update freely (there have been a number of fixes to epgp since MoP and you probably want them).