I'd walk away. Martin has incredible talent but he has never shown the consistency required to be worth a top 10 pick in a trade and he is 24 now - can't just rely on potential. I think 15-18 is probably about right actually.
Stringer went for 25 and 30 and was a far better performed player, though that trade was mainly just the dogs wanting to get rid of him so its probably a reasonably cheap one.
Initially I was thinking Walker & the 2nd round pick we got from the Suns (20ish?), maybe that was overs (forgetting the fact that the Suns might not want Walker).