Its not such a bad job if you have worked with PVC before.
I did a similar job a few months back although the run was a bit shorter (about 2 feet) when snaking out my bathroom sink like I usually used to do every 3 months when it would clog. This was due to the galvanized pipe.
My snake actually broke through the threaded end of the galvanized pipe where it met the 90 degree elbow and water came pouring into the basement.
So I ripped the wall open and replaced it all with PVC. Check out the inside of the old galvanized pipe.
Well theres your problem
!!
The galvanized pipe threaded into a cast iron pipe via a oakum and lead hub fitting on the top and bottom with a threaded T on the side to accept the galvanized of course the pipe wasn't just going to turn out of there after 50 or so years.
First I removed all of the pipe I used a reciprocating saw and cut about 4 inches past where the pipe threaded into the cast iron T and removed the rest of the galvanized to the sink.
Then I got out a big 3 foot chisel and a 4 pound hammer and pounded a hole in the pipe right where it met the CAST iron T. After the chisel went through the pipe I used a monkey wrench and pressed down to cave the pipe in on itself.
After that I simply took my water pump pliers grabbed the mangled pipe and wiggled a bit and the end of the pipe came out threads and all.
From there it was simply a matter of buying a threaded to PVC glue fitting. I used the fitting that usually glues to the end of a PVC sink trap and connects to the tail piece. However according to HJ you must purchase a special threaded converter for this purpose while has thicker threads.
I coated the threads of this adapter fitting with liquid teflon (in a can with a brush applicator for sealing gas pipe threads I think) since I feared that the rough cast iron threads would tear up teflon tape like tissue paper. I inserted a short length of unglued PVC pipe into the fitting to turn it into the cast iron tight then removed it. From there on I simply glued the PVC to the adapter fitting and installed the piping to the sink.
Needless to say I haven't had to unclog the drain since December when I did this.
Probably since PVC doesn't have that rough inside (which later rusts) like galvanized has which catches all of the nasties that plug up the drain.
I also did the same for my bathtub which was actually in worse shape with visible rust on the outer wall of the pipe!
As for your question yes $800.00 is reasonable for that repair.
Although if you know how to do it yourself the parts will probably be under $100.00.