I picked up everything I needed except the drive shaft for $800. Don't remember how much the install was. That was with 3.45 gears, so you would still need to figure in the price of the 3.91 gears and installation.
I think you should really do some research beyond price. 3.91 is a lot of gear for the V6 A6. You will have to shift to third just to get up to 70 MPH and you will be running about 2200 RPMs at 70 in sixth gear.
EFR Twin Turbo LFX-GPI Tune-ZL1 fuel pump-10:1 CR forged pistons-3.45 gear-Meth Injection-BMR Trailing Arms, Bushings & Sway Bars-CircleD 4000 Stall-GPI Fuel Enrichment System
647 RWHP & 726 RWTQ @18.5 psi on 93 Octane (locked converter)
1/8 mile -- 7.158 @ 102.10 (20psi); old build