That is a very good price for the warranty as long as there are not any strange exclusions in the coverage.
We have many expensive electronic components/modules that do fail in these cars. Even if you do not have any major engine/transmission failures, I believe the warranty would likely pay for itself during the 6yrs or 100,000 miles. I am basing this the $$$$ I spent during 30k ownership of my 06 M coupe.
Good piece of mind if you don't plan on heavy modification and will not be doing the majority of the wrenching on your car.
I do agree that Carmax pricing is typically high and values are all over the place on these cars. In my opinion, find one with the color and options you like that is in great condition and grab it quick
Get the best deal you can, but don't get so hung up on price that you pass on a good one.