I certainly haven't been able to find anything 'official', but our group has settled on 12 XP. It fits with the Sigmarite Warrior-Priest, who has a very similar stat line, and the Priest of Taal. Given both of these are official models, I think it's the best benchmark.
I can certainly see the argument for 8 since it replaces a champion, but 12 seems to fit better given the other priests.