Originally Posted by Wolfador
on the e39 5 series it is about $700-$1000. Depending on what you want to replace.
You can either change it all in once (waterpump, thermostat+housing, radiator, expansion tank, hoses) or just wait until something breaks and fix it one step at a time. If you have the money, the first option is a good one. I've had cooling system parts break twice on my E39 (first the t-stat housing and later the t-stat itself) and it cost me a lot of money. So preventative is better. But you should also not sweat it that much. Just keep a good eye on your coolant level and your temperature gauge.