$200 bucks? If the rear has been rebuilt, I'd say it is a fair price, but if it was just pulled out of a wreck, no that's too much. I just called a boneyard a couple of weeks ago, and they charge $50 bucks if I pull it out and $100 bucks if they pull it out of the car.
7.5" GM G-body non-posi rearends are all over the junkyards...it's the 8.5"s that are hard to come by and cost an arm and a leg. Some in depth searching for a 7.5" with a posi unit is going to be required on your part. It'll require a little more searching if looking for one with 3.73 to 1 gears. I would search for SS Monte Carlos', that's your best bet. Do you know how to do a quick check for the ratio? Mark the tire or brakedrum (piece of chalk will work) turn the tire or brakedrum one complete revolution and count how many times the driveshaft, yoke, or pinion (whats ever left under the car) turns. If it turns about 3 3/4 times BINGO. You could also check the RPO codes listing the cars options if you can find the RPO sticker (check in the glove compartment)...my manual shows the RPO code as GT4 for a 3.73 gear ratio, although there are a lot of different codes (it would be best to have a manual with you when hunting). You could also have the rearend currently in the car converted and or rebuilt using 3.73 gears and a posi carrier, but this will run you some bucks for parts and labor. Remember part of the fun is the hunt, especially when your successful! Remember any 78-87 GM G-body is a possibility. Good Luck