MPX mags are a little on the pricey side, but then I point at what STI mags cost and the pain goes away.
What length is your pistol? I can't seem to make up my mind what to go with. Gen 1 had 4.5, 6", and 8" pistols. Gen 2 is 4.5" with some 8" available through LE sales.
I did run a factory 4.5" MPX SBR at the Sig academy range a couple of months back for a CQB course. I was very impressed with the accuracy out of that short barrel, but found the handguard a little too short for my liking. The most comfortable position ended up with me grasping the magwell with my thumb over the top rail.
With that experience in mind, thinking about setting up a 4.5" barrel with the 8" handguard and just keeping it suppressed. Or, go with the 8" barrel and only occasionally suppressing the MPX. I keep going back and forth on that point. The suppressed 4.5" is bad to the bone, but unsuppressed for the the occasional PCC match would be more practical.
I'm going to bite the bullet before end of the year one way or another.