I bought my stainless 4" GP100 new for $550 around 6 years ago. It sounds like a pretty fair trade to me. BTW, the GP 100 is a fine gun - built like a tank, nice trigger in both single & double action, shoots accurately & looks great.
Great trade! I've been thinking about adding a .357 at some point too. I've looked a bit at the Ruger GP100, and it seems like a great gun. I would agree that the value is about the same for both guns, so you and your friend probably both came out fine. Even if there was a slight price difference, it's always nice to trade something you don't want for something that better fits your needs. I figure if it doesn't work out exactly evenly on paper by doing so, yeah well, so be it. Everyone is happy.