Check you owners manual under maintenance and see what the manufacture recommends when to change it. I would shop around and find someone who is cheaper or a mechanic who does it on the side, you will pay half that. I know its expensive because of the labor that's involved. I have an 04 and I work on my car and it is a pain to work on that's why its so much money. In fact I have the same oil leak which I have to take care of also, this common for Regals. The problem is how the made the oil pan its not flat and that's what causes it to leak. An oil pan job is around $400 and the transmission is about $100 so he is in the range.
I hope this helps, I recommend to shop around to get the best price!