What's amazing is that if the prices for codeshares were always exactly the same, people would be screaming "price fixing!" as this could of course only be done with coordination between the carriers. In fact, this is the reason that fares are often not the same on codeshares. The airlines are prohibited from working together to keep prices on codeshares the same.

While it is true that they sometimes move seemingly in lockstep, this is accomplished by one airline responding to another's changes in their published fares. Whether this occurs or not probably has very little, if anything to do with codesharing, and everything to do with what each airline's pricing model tells them is the best thing to do.