Well, here is my opinion:
If you have NOTHING, IE a bare engine bay - 950 for everything to drop the motor in, turn the key and it runs - is not bad. All of the small supporting parts will nickel and dime you to death, and depending on how much you value your time - it may be a good deal. Does that include the transmission as well?
No used motor is really worth a whole lot IMO. Unless I know the guy personally, I always assume that it is basically a core and any service I get out of it before having to rebuild is a bonus. If it's got a bunch of aftermarket parts, then I factor those into what I think the core is worth.
I've seen way too many people who bought a "good, running motor" off craigslist that turned out to have all sorts of problems.
If you are trying to restore the car, don't feel particuarily excited about chasing down all of the relavent parts, value your time highly, AND are willing to take a gamble on the true condition of the motor - then I think this is a reasonable deal for you.
If it were me, I would probably take a little more time and find someone who is ditching their I6 for a V8 and buy ALL of their little parts, buy a junkyard late model 200 - and then rebuild it myself.
In that scenario you would probably be in the same ballpark $$ wise, but would end up with a fresh motor that you knew what the deal with was.
Of course, that approach takes a lot more time an effort.
As far as cost of rebuilds - I guess if you drop the motor off at a machine shop and say "rebuild this" you could spend $1500-$2000. If you buy the parts yourself and do your own assembly, only paying for the machine work (deck block, bore, touch up crank if necessary, valve job) your going to be in the $500-$600 range.
It really depends on what you do yourself, and what you ship out.