especially if you're lowering because then you need adjustable arms front/rear and adj tension rods (or can sub the 300degree camber kit up front).
Lowering requires the ability to set everything to proper caster/camber/toe so you track correctly and don't wear the tires off unevenly. So don't forget to include the cost for a decent installer who knows how to set things up properly for your intended use.
My Powertrix SS install with labor was around $2300.