Best Bang for the buck IMO is a magpul rear mbus if you're OK with it being polymer.
Matech mill standard BUIS. Got mine NIB on amazon for $64.99. Not a bad deal for a great product.
I own a Troy BUIS and a Matech. Best value for the dollar goes to the Matech. The Matech has simple 100 yard elevation adjustments out to 600. What I like better on the Troy is the lockup on fold down/up.