For only 4.5 million more, there's no reason why they shouldn't have gotten a deal done, but I don't see this trade being that bad for the Thunder.
Biggest reason being I'm a fan of Jeremy Lamb. I think he can be a solid scorer, but I think he has huge defensive potential. Offensively, there isn't much of a drop off between Martin and Harden. Aside from last year, Martin has always been an efficient scorer. As the 3rd option on the Thunder, I can see his efficiency going back up. Obviously he's not the passer that Harden is. Or the defender, but Lamb should make up for that, eventually becoming a better Sefolosha for the Thunder.
The draft picks they got are rather worthless though if I'm reading right. Lakers first round pick, worthless. Lottery protected Raptors pick isn't all that great either. Like I said though, no reason why they shouldn't have gotten Harden locked up though.