The big problem a MMO faces is getting the same voice actors to return for later content updates. How many people were annoyed when the voice acting was different for specific characters between ARR, HW and SB? If they couldn't get all the current voice actors to return to do voiceover work for the patch, would you have been happy with substitute voice actors who sounded very different?

While voice actors get paid fairly high by the hour, the number of hours required isn't that high so the cost isn't that much when compared to the total game budget (or expansion). I doubt money was the issue. Being able to get all the voice actors available to do the work on relatively short notice probably was. It would have been awkward to have some voices the same while others differ.