If the tank, fenders, and seat are in good shape, you could probably sell them for more than $200. I used to wrench on Hondas, only worked on 2 Nighthawk 650s. They were good bikes, nothing fancy, and were a pretty good seller. There were three versions of the nighthawk series, the later ones were really slick. If my memory serves, the top ends did have some issues. They leaked oil aournd the head gaskets, and I believe they had some problems with soft splines on primary socket. The black on the motor flaked after awhile, the later models had black pipes and they flaked off pretty fast.
That is a lot of miles for any bike, especially one that old. If there is a problem in the top end, it will cost a lot of money to fix. Figure about 20-30 bucks an hour for mechanics labor plus parts. Problem is, if you fixed the top end, it would probably only be one thing out of many that need to done. All it takes is a bad starter to make you realize parts are very hard to find for those older bikes. Add tires and odds and ends and you can get to $1000 pretty fast.
Personally, I wouldn't buy that bike unless it really looked good and you were comfortable with it. If you keep looking, the CB Honda line is good place to start for low price bikes. Also the KZ Kawasakis and GS Suzukis from the mid 80s are good bikes and very cheap. They are all pretty close to each other: inline four, air cooled, some were shafts. I highly reccomend a shaft bike. Look for ANY rust in the tank, make sure all electronics work, and listen to the bike at throttle peak and idle. You want to hear if the carbs are synced up right. The carbs are the achilles heel of those bikes, if they are out of sync it is very difficult to find somebody to get them right. My father had a Suzuki GS1000G that he bought new in 1981, I know that guy that bought it and he still rides it with about 80,000 miles on it. I just recently sold my 1987 Honda Shadow VT1100C with 128,000 miles on it. If you take care of them, they will do the job for you. If you need any help, PM me and I will tell you what I know, which may be of value or not.