Or go to a different dealer! If you're not willing to join Mazda Motorsports, you'll pay 25% more and get one from Rosenthal.
Of course, if you're buying cars for $53.85, you're going to find there are a lot of things out there that cost more than you've paid for a car. Like a tank of gas
Is that the MSM part? I'll order it today if so!
I think you need to speak to the people actually selling the parts instead of some guy with Google open in a browser Tommy at Rosenthal knows Miata parts really well.
I suspect that the MSM part is only slightly different than the non-MSM one. It's probably got a couple of holes cut in the side for the IC pipes. The same sort of holes that every single aftermarket turbo installer has to cut. I actually installed a non-MSM undertray on a V8-powered MSM yesterday.
If it were my car, I'd do it. All of my Miatas have undertrays of some sort, although one of them looks suspiciously like a thick alloy skid plate.