basically, +1 per 4 levels, rounding up.
non + items in the same price range as the + x items
I think it's dependent on your world/campaign.
My current GM gives out underpowered gear and has since time immemorial. But it's ok, because our characters are awesome not because of the gear they use, but from their abilities combined with our wile.
If your party is geared out, just make sure the enemy is also geared out to provide them a challenge.
my 2cp.
Go look at Automatic Bonus Progression.
Roughly that.