A DIY expansion tank replacement on a bmw x5, applicable on several other model vehicles as well.
Unfortunately, coolant tank replacement is a common occurrence on these vehicles. The tanks typically begin leaking through the weld on the plastic near the top of the reservoir. Coolant expansion tank replacement is typically needed every 60,000 miles.
The good news is that coolant tank replacement is relatively quick and simple and well within the scope of any diyer. 
There are two hoses that need to be unattached, and two 10 mm bolts that need to be removed in order to remove the old reservoir. I recommend replacing the cap as well, as these are a failure prone component.