a regular style high performance thermostat is usually 8.99-19.99 if you buy an aftermarket source. The special ones (built into the housing or pipe) could be lot more. (look at the Range Rover) By looking at the picture this is a standard type, not something special. I also searched the ETK CD and looks very simple. The 840Ci isn't much different but in temperature it might be different... Anyway it is a lot of money for a thermostat, but if you think how much it saves you then it is a bargain ! To replace them every 4-5 years that seems a little to often but depends on what quality they are. I have seen thermostats going for 100k miles and also failing at 12,000miles.