I've got a 1970 Sonic 1 head that I got off ebay for $70. It had some issues with it and the face plate had all rubbed off. I fixed the issues and rebuilt it complately, and made it into a 200S with new labels on the front, as that is essentially what it is anyway. As far as what its worth now, I have no idea, but I'm not inclined to sell it so it doesn't really matter to me. I would say though for your amp since it sound slike it is in nice shape is that it would be worth around what a nice 200S would be, so you may check ebay's completed listings and see if you see anything there to help you gauge the price.
greg