I think it depends on where you are in the world.
Put it like this, a new 335i with options over here in the UK is £40k ($80k) trade price on a year old one with 12k miles on it at the moment is around £25k ($50k).
Discount on a new 2009 model is around 6% at the moment so you would get it for say £37k, so cost to change for an 09 model is around $24,000.
Adding the new iDrive for a grand or two really doesn't seem so bad to me.
