Hey font2010.
My late 1970's Yamaha literature is far from complete, but I'll let you in on what I see.
The 1977 catalog lists the SF 500, 700 and 1000. The 1980 catalog does the same. By 1981, the SF series is gone.
So, I'm thinking your SF 400 is pre 1977 but I don't have the literature to pin down the date exactly. I think I may have more late 70's Yamaha literature that hasn't been posted to the Catalog Library yet and I'll look for it when I get a minute.
As for value, the SF series isn't listed in the Blue Book so you'll probably need to search the web for examples that are listed for sale.
Hope this helps