This. The Five-SeveN and P90 are

-Lightweight
-Compact
-Easy to wield
-Large ammo capacity (Five-SeveN = 20/clip, P90 = 50/clip)
-Good range (P90 = 150m, Five-SeveN = 50m)
-Good rate of fire (the P90 can put out 900 rounds/minute)
-Accurate
-Low muzzle flash/recoil
-Both weapons use the same ammunition
-5.7x28 cartridge has armor-piercing capabilities (for hostile survivors), and create large wound channels (for zombies).

On the downside;

-Expensive
-Five-SeveN not in widespread use
-Slightly less effective against unarmored targets

I did some research on Google...
Doesn't look like you're getting a (new) P90 for under $1400. And you're probably not getting one anyway because it is not sold to civilians at all. (In the US, anyway)

The Five Seven seems to be about $900. I think you can actually buy that one, depending on where you live.

I don't understand why people want so many military-grade, super-powerful guns. Hell, I'd take a .22 rimfire 'cause it only needs to get to the brain, pretty easy at medium range, plus huge amounts of cheap ammo. But I'd prefer any type of Remington hunting rifle, semiauto so I don't waste ammo, widespread cheap ammo, and a good range. Preferably with a scope and some type of long-life battery powered laser.

Holy fucking balls.

I knew it was expensive, but that's crazy.

Because they haven't read the Zombie Survival Guide. M1 Carbine and Shaolin Spade (though you'd have to buy it under Monk's Spade these days) for me.