Anyone know what the cost of rear brakes would be if done by a mechanic?

Depends on what needs doing and which pads you want in there. (cheap pads are $40, good ones are $80 plus fitting)
If the discs have too much run-out or are deeply grooved they will need machining (around $30 each or $60 for new ones).
The caliper may need overhauling (kit is $30 plus half hour of labor each).

I just done my rear brakes and the guide pins were sticking causing a squeal. I also adjusted the handbrake while I was there and flushed the brake fluid since it was due for its 2 yearly change. I found some good rotors at the wreckers for $21 each and done my own machining at work.

So for a complete job you would be looking at $200-$300. You may get away with just replacing the pads which would come in under $100 inc labor.

Ok thanks for the info

Do you have an idea how much it will cost to replace rocker valve cover? If done by mechanic