PRices tend to be same over both formats. Any price differences will most likely just be because of price matching at the current time.
As it stands the ps3 version of AC3 is the definitive version along with 1 hour exclusive content you gain further unlocks if you also have the Vita AC3:Liberation (female assassin who meets Connor at some point).
From what I've been told the PS3 games are more expensive because it's harder to code games for the console. What I've HEARD in rumours is that Xbox have exclusive rights to certain games first release of all the DLC so will drop the game prices to make sure they sell faster than the PS3 versions and release the DLC first as an incentive to make sure players choose Microsoft over Sony. Whether that's true, I couldn't say. When you look at games like Skyrim and see that nearly a year later, no DLC has come out for Skyrim on PS3 but HAS about 3 or 4 for Xbox already, you can't help but believe something is underhanded about it.
Several PS3 pre orders have been around £5 more expensive than the 360 versions in the past, I've emailed Amazon to ask why and they've told me they price items due to the manufacturers recommendations but I've never heard of games publishers etc recommending that PS3 versions be more expensive than the 360 versions before!!