Firstly because he is new, his price is going to be high. Second, he is a support but he is better than Iruka. He has the potential to be very powerful alongside Kakuzu, Rinnesasuke, and GNW Madara and Shark Kisame when those two get their BT. He arguably has a better kit than Edo Itachi, whose best properties are his mystery and status reflect. Itachi's standard is nothing special and he doesn't cause any useful effects outside of dream (he does ignite but so many ninjas are immune). Pretty much every one of Suigetsu's skills are good: shields, dodge, unmissable interruption PLUS +1 to the opppnent's cooldown, stackable defense decreases to the opponent, stackable attack+nin boosts for your field (the whole allied field, not just your team like with Iruka).

I still think he's a bit overpriced, but things in general are overpriced. But that's why they priced him like this. Over time he'll become easier to get.