This past local show had one NIB in 4" at $650 !
I'm not surprised. If I had one, new in the box...truly unfired since leaving the factory, I'd probably ask that much for it too. How many of them are there in that condition? Start high. You can always come down, but you can't go up.
They were/are good guns. Back in the day, I had two or three of them, can't remember which now. I know I had two, a Speed-Six with a 2 3/4" barrel, and a 4" stainless steel Security-Six, but I seem to remember a blue 4" Ssecurity-Six in there somewhere too. I traded/sold them off, lost the SS one to my ex when we split up and she claimed it as "her gun." Oh well, I had others.
When Ruger stopped making them, and bought out the GP-100, I stopped buying Ruger centerfire guns and haven't bought one to this day. I just never cared for the GP for some reason.
Now to the OP: That's a nice one you got there. Sort of a shame it's been refinished, but they were made to worked. Lord only knows what it may have looked like before. Now you don't have to worry about hurting the finish, although Rugers were always pretty tough in that department.
If you can find a T-grip for it, it will help those grips a lot. We used to joke when Ruger went into business he bought one tree to make grips from...and he was still using it. If I couldn't find a T-grip, I'd just get a set of Pach's for shooting.