1975 S3 400cc Value?

OK, so I'm still pretty new to the old Kawasaki thang...but learning and loving it. I have the chance to buy a pretty clean and original 1975 S3 3cylinder 400cc bike. It runs real well w/ about 12K miles on it. It's complete and all original. Fairly fresh tires w/ lotsa tread. I have no idea what it's worth, so i don't know what to offer. Here is a pic, please lend me your opinion. I think the seller wants about $750 for it. Clear title. What's the value?

Jason
Oklahoma