The P90 AR is so small it doesn't need a separate sidearm! LOL!
The CCI 22 WMR rounds they recommend are $10-13 for 50 rds and the FN 5.7 x 28 go for $19-23 for 50 rds where I live.
OK, so the price difference for ammo isn't as big as I thought, which is good. Though as you point out, the difference in weapon price is also double.
What sort of usage does the FN 5.7 enable that the PMR-30 can't match?
The FN 5.7 x 28 has armor piercing capability and much greater lethality than the 22 WMR due to its extremely high muzzle velocity which translates to greater muzzle energy. Expandable bullets would be particularly damaging.