I wouldn't do over $600. 'Basket cases' everyone thinks refers to the parts in separate containers, it doesn't. It means, "bring baskets of money to complete this forlorn attempt at finding out what was wrong w/this abused pile of crap in the first place!" That's why it's apart, he had something seriously wrong and tried to discover what it was. Once he found it's gonna take $5,000 in shop parts & labor to make the pile rideable again, it stayed disassembled.
My view: if he's such a good mechanic, why didn't he finish it? If it's such a good deal, why didn't he reassemble it and sell it for more $? Don't walk, run, run away from this, unless you have whatever you're going to spend available to not get back. Unless it's got lots of good shape aftermarket parts which you could piecemeal sell, and junk the rest. Or, take the extended time to sell stuff on here for 25% of what it's worth, just to get some $ back.
My opinion, it's always better to buy an entire bike together, because it's pretty obvious what's there and what isn't. Even if you need an entire engine to swap, that's $800-1200 depending on the age & miles, though you'll find hosers trying to sell engines for $2K. Buying a basket case, it's easy to overlook some essential parts which are going to be expensive to replace, like the pick-up coil(s, if it's an '85-'89), the engine CDI, the VBoost electric controller, the fuel pump relay (check the current over the counter price on one of these!) replacing cracked intake manifolds (easy to break if you use an impact driver to try & remove them), the VBoost butterfly, not showing anymore as an available dealer part last I checked, and then your whole issue of, "is the engine solid or is it solidly locked-up?" And most importantly, the title. I would never buy a bike w/o a title.
I've bought basket cases but I paid even less, because some of the essential parts weren't there. I already had inventory to replace them, but that additional cost must be figured into your purchase price. Stator, rotor, reg./rectifier? An easy $300+ at good used prices. Rebuilding the carbs coud cost > the bike, using factory replacement parts.
So, don't be too quick to jump. Sellers are motivated by cash. Offer $500, and have it to do the deal, and a way to transport the bike on the spot. A $1,000 basketcase is not a good deal, unless, as I said, it's got lots of aftermarket parts, and you can afford $1000 for a replacement engine when it's got the #3 conn rod/bearing & etc problem & is toast.