edit: woops...I dunno how I replied to this old thread.
First, Do you hear any noise from the rear brakes? The shoes will usually make a scraping noise when they're low but it's a bit different sound than worn brake pads on a disc brake.
If you don't hear any noise you're probably fine. But also consider how many miles are on your car and the driving conditions. Does it have a lot of miles and/or has it been used in lots of stop & go traffic? If the car is 10 years old and has 150k on it, I'd probably at least take the drums off and inspect everything. As mentioned above, drum brakes will last longer than disc brakes, but exactly how long depends on a) the driver/driving conditions and b) the engineered front/rear brake bias of the particular vehicle.
I recently planned to replace the drum brakes on my mom's '00 Camry. It's got 100k miles on it. I took off the rear wheels and drums and was amazed to see that everything looked pristine. The pads were maybe 25% worn and there was zero rust save for light surface rust on the drum, despite this being a northeast car. So I just put the drums and wheels back on.
In any event, if you want to do the rear brakes, all PARTS shouldn't cost you more than $100-$150...that's aftermarket. I don't know what OEM parts would cost, but that would include drums, shoes, and all hardware (springs, adjusters, etc). You'll also want to make sure that the wheel cylinders aren't leaking. If they're not, you don't have to replace them.
A auto mechanic is going to mark up parts, so expect to pay more. Labor? I'd expect in the $100 range, but ask around.