Ok, so I'm posting for a friend that's looking to get into bass.

He's basically already set on a Peavey Mill, but since I no knowthing about bass, we're kind of stuck on amps...

I play guitar, and he wants something nice but that doesn't cost an arm and a leg. We'll be jamming together, but no gigs or anything like that.

Play mostly metal, metalcore. Looking to spend maybe 400-500ish USD.

All Hail! The Kala-Kala Chieftain!
Look at the Ashdown Mag series, Fender Bassman, Nemesis silver or the Warwick Sweet.
All of those are fairly good rigs (combos mind you). Loud enough to jam or even gig. Just stay away from Behringer, and some say GK Backline/Goldline are bad too but i don't really have experience there.
Now, a question of etiquette - as I pass, do I give you the ass or the crotch?
I second the Ashdown MAG. I have the head, and let's just say it's amazing.
I've played through the Bassman 250/210 and also the Ampeg BA 210. Both are nice. I own the Bassman so that's the one I use. I've heard the Ashdowns are nice. I'd like to try one but no one around here carries them.
Fender Geddy Lee, Precision (MIA), Fender Standard Jazz, Ibanez SR700, GK MB210 combo, GK 210MBE