If I'm Presti, I'd give up anyone on my roster not named Durant and Westbrook to get Howard, assuming Howard will sign an extension.
Harden, Ibaka, Perk and a pick should get it done. OKC can play Collison, or sign a stretch 4 from Europe to play the 4, if PJ3 can't. Durant, Westy and Howard would be the best Big 3 in the league.
Regarding OKC being a small market, Durant is one of the NBA's most marketable stars. KD made $11m in endorsement cash in 2011. Howard made $12m in Orlando. Being in the Finals every year and winning rings will shoot both those numbers upwards.