Generally, I'd say if the car is in excellent condition, with normal mileage per year, I'd say depreciation would be around 5-10% per year. That is, you lose 5-10% of what you originally paid for it per year.
Say your E90 cost $40,000 USD. In the first year, if you have reasonable mileage (less than 15,000 miles is average, I think) then I don't think it would be a problem for someone to buy the same car for around $36,000-38,000 the next year--you know low mileage, only a year old, etc, it would still hold its value.
Maybe those figures are a little high or maybe they're right in line. But the point is condition/mileage is important and will ultimately decide most of the depreciation.
You can go by this 5-10% drop per year for a while, but eventually it will level off and actual depreciation could actually only be a few hundred dollars per year. Think about it, at 10% per year, if you follow this strictly, then that means the car will be worth nothing after ten years and we know that certainly won't be the case, if the mileage and condition are both in line.
But for all intents and purposes, at least for the next 5 years, I think 5-10% per year is a good estimation.