This is how $400 P-10 Cs with 10 round mags are a good deal. You already have one P-10 C with 15 round mags and you want another one as a back up. You shoot IDPA CCP where 8 rounds is the maximum allowed in a magazine.
If it is your first P-10 C and you live in a high cap state, then it is not better than spending $440 and getting two 15 round mags.
I have two P-10 Cs. The second one I got for $380 with two 10 round mags. I am very happy.