Maybe I need to move to a different part of the country. Here, a pristine 2.75" SS Security Six will bring close to $600 (probably from me).
I picked up a blued 6" Security Six LNIB for $250 plus tax in 1990. I still regret letting it go. A couple of years ago I bought a Service Six from a family member and I now am on my fifth Six series. The cheapest one came in at around $372 (including shipping and transfer) for a 6" SS in $85% condition. I got really lucky with that one. A 99% 6" SS w/boxes came home for over $625 and I feel it was still a fair deal.
Most recently, a 2.75" SS Security Six (98%) followed me home for a little over $500 including tax. I still can't believe that shop sold it for so little (it was on the shelf for half a day). The 2.75" stainless is the most popular and the actual selling prices on auction sites reflect it. When you add shipping and FFL transfer, the auction prices are real eye-openers.
Offer your friend a fair price (by today's standards), and you may end up with the most durable alternative to a K frame that was ever made. In today's dollars, you can't buy this unique blend of quality, fast handling, and ruggedness in anything new for anywhere close to $600.
The prices will only increase because people like me now know how good these revolvers really are. While I like the 2.75" and the 4", the 6" is the best handling (CCW problems though). I plan on picking up more of all lengths in the future