I bought a stainless 4' Security Six last Saturday for $350, and considered myself lucky to get it. I've been looking for Ruger Sixes for quite a while now. I sold mine back in the 80's, and have been kicking myself ever since.
The shop actually had
two Security sixes, but they kept one for themselves!
I consider the Security Six to be a better gun than the GP-100 - better balance, better trigger, etc.
In fact, I recently tried to trade a new stainless GP-100 for a blued Security Six. I was turned down.
To answer the OP: $300 is a steal for this gun. If you check Gunbroker, you'll see them going for more, plus FFL costs and shipping.