What about reliability?
I hate to burst a balloon, but a custom build is not what determines reliability. The number one cause of malfunctions in an AR is the magazine. A top-dollar rifle with gun show GI magazines is not going to be reliable. The second main cause is improper cleaning. Run the gun wet with lots of lube in the bolt carrier with good quality magazines and any AR, custom or not, will purr like a very loud kitten.

The main benefit to a custom build is—and I’m being dead honest here—the cool factor. When you unveil a decked-out custom build, embellished with a Schmidt & Bender “Flash Dot” scope, a SureFire WeaponLight vertical grip and all the other “tacti-cool” accessories, you’re going to turn heads. That’s the one thing I can guarantee with a tactical build.

The Bill, Please
How much does a custom build cost? The answer varies depending on how extensive you want to go. Assuming you want all the bells and whistles, you’re looking at $2,000 for a new trigger and upper barrel assembly with a top-grade barrel. If you don’t already have a high-speed stock like a MagPul, SOPMOD or Vltor, add two bills.

If you want to use your factory barrel and go for a new bolt carrier group and the same Vltor VIS upper assembly that would come on a full-house build, the tab drops to a little over $1,000.

Here are some sample prices, picked from brands I trust, as a ballpark for pricing.

Vltor VIS upper assembly                         $660 (with fore-end, less barrel)
Vltor MUR upper receiver                          $230
LaRue Stealth upper receiver                    $250
Noveske Recon upper barrel assembly        $1,295 (includes BCG)
MSTN SAM upper barrel assembly              $1,795 (includes BCG)
Noveske 16” Recon barrel                         $580 (with low-profile gas block)
BCM bolt carrier group:                            $150
Geissele trigger                                      $170
Wilson trigger                                        $270
JP trigger                                              $140
Timney trigger                                       $195

Another way to go is to buy a “custom” rifle from one of the better builders. Of course, stay leery of any that claim to be Mil-Spec.  If you look at “high-dollar” ARs with the idea you’re getting a quality, custom out-of-the-box AR, make sure the company isn’t hyping something that’s impossible to produce. I repeat, there is no such thing as a Mil-Spec rifle, component or part—not even a screw—available for commercial sale.

For more, check out AR-15 Tactical Accessories, AR Operating Systems: Gas Impingement vs. Piston, The Specs of MilSpec and The MilSpec Definition.