Rather than financial, I look at it more from a Duncan standpoint. Duncan has a limited number of years left and there is not much chance another Duncan is on the horizon.
I think the Spurs go for a last hurrah; they can be conservative once Duncan retires.
Manu's recent comments about exploring all options this summer is also a motivating factor for the Spurs to trade him now. If not, they run the risk of losing him for nothing over the summer.
Personally, I'd be very surprised if Manu isn't moved prior to the deadline. If he isn't moved, I'd guess the Spurs are very confident in re-signing him in the summer.