Nice machine! What you paid for it is about what it's worth, in the knee control cabinet maybe a little more. Case or cabinet really makes no difference. If you can sell it for more, then it's worth that much. if you had the manual and the accessories, then a little more. It's a common 15 class clone, there are literally millions of them still in use so it's not rare or special other than the fact that it's a very good machine. Sovereign machines were built and sold by Brother in the mid 1950s.
The clones are not prized because there are so many of them, blue is the most common color after black. They only do one thing but they do it better than almost any other machine, they would sew a bumper to a Buick if it you could get one under the presser foot.
Now, did I give you enough conflicting information? lol. I'll summarize - very good machine, not worth a whole lot.
Cari