Use the email linked to your current Battlenet account for the RAF's and the new accounts will be added automatically.

It cost me $59.99 per account to go from RAF trial to MoP and this was about a month and a half ago.

I missed the fact that your highest lvl character was an 85 Priest the first time I read your post. You have anything else close to 85 that you could level to boost with? Lots of different options on what to use as a boosting class but I think a priest would be near the bottom of my list.