http://us.battle.net/wow/en/blog/2943351#blog

In Short:

Your Valor Points + Justice Points can't exceed 4000 when the patch comes out. If it does, the amount of leftover Valor Points will turn into 47silver per point...

So if you have 2900 Justice Points and 1200 Valor Points, you will login with 4000 valor points and (100 x .47=) 47 gold.