Check Gunbroker... but also check Armslist. I follow Six-series prices very closely. Gunbroker prices are high, but supply is also low. On Gunbroker the stainless guns seem to bring a bit of a premium. Not really sure if the 2.75" or 4" is more popular right now, I see more 2.75" auctions, but historically the snubnose is more popular. Also it matters if it's a .38 or .357. The .357 seems to be worth a little more.
The Security Six I see a lot more often than the Service or Speed Six.
Assuming it's in excellent condition and is a .357, I'd say probably $500-$550 on Gunbroker. Your local market can vary widely, which is why I recommend looking on Armslist. Six-series revolvers seem fairly scarce on Armslist, and rarely will you find one in your state necessarily, but the prices on Armslist seem to be about $400-$500 depending on condition.