Originally Posted By StrangerDanger:
Big time money pits. Like a Harley brand boat. 

A client of mine is having me build exactly this, the ultimate M4. Here is the roughed our pricing and links to parts:

-Benelli M4 Purchase, 11721
-Replacement Rear Sight Screws x2: 17.99
https://www.brownells.com/shotgun-parts/sights/sight-parts/rear-sight-parts/rear-sight-screw-sku301000542-23694-50302.aspx
-Scalarworks Sync Rail: Supplied by Owner
-Trijicon SRO Optic: Supplied by Owner

-Custom Fit Limbsaver 10403: 80.00 
Back Ordered due to Corona. 5/10/2020
https://limbsaver.com/products/classic-precision-fit-recoil-pad?variant=42673926675

-IWC Benelli M4 Light Mount: 66.50
https://www.impactweaponscomponents.com/product/multi-light-benelli-m4-mount-scout/

-Surefire M600DF Weapon Light: 244.00
https://www.impactweaponscomponents.com/product/surefire-m600df-scout-blk-1500-lumen/

-Modlight 18650 Light Body: 74.00
https://modlite.com/collections/light-bodies/products/modlite-18650-body

-Blue Force Gear Padded QD Sling: 100.85
https://www.blueforcegear.com/custom-padded-vickers-sling

-IWC MOE QD Rear Sling Mount: 28.45
https://www.impactweaponscomponents.com/product/qd-rotation-limited-buttstock-sling-mount-n-slot/

-TTI Stainless Bolt Handle: 54.34
https://tarantacticalinnovations.com/tti-benelli-m4-ultimate-charging-handle/

-GG&G Oversized Bolt Release:  126.94
https://www.brownells.com/shotgun-parts/bolt-parts/bolt-hardware/benelli-bolt-release-button-prod26414.aspx

-Tooltech Night Sights, installed into NP3 plated apertures: 250.00

-A&S Engineering Trigger Frame: 219.00
https://freedomfightertactical.com/collections/benelli-m4-tactical-shotgun-parts-and-accessories/products/a-s-enhanced-trigger-guard-benelli-m4-922r-compliant

-FFT Trigger/Disconnector/Hammer Kit: 264.99
https://freedomfightertactical.com/collections/benelli-m4-tactical-shotgun-parts-and-accessories/products/the-freedom-fighter-tactical-922-compliant-trigger-pack-no-snap-ring-pliers-for-the-benelli-m4-shotgun

-FFT Shell Elevator: 89.99
https://freedomfightertactical.com/collections/benelli-m4-tactical-shotgun-parts-and-accessories/products/the-freedom-fighter-tactical-worry-free-extended-carrier-for-the-benelli-m4-tactical-shotgun

-FFT Wolff Trigger Pack Spring Kit: 39.96
https://freedomfightertactical.com/collections/benelli-m4-tactical-shotgun-parts-and-accessories/products/new-the-freedom-fighter-tactical-wolff-gunsprings-benelli-m4-trigger-group-spring-kit

-FFT OEM Reproduction Handguards 69.99
https://freedomfightertactical.com/collections/benelli-m4-tactical-shotgun-parts-and-accessories/products/the-freedom-fighter-tactical-922-compliant-forend-for-the-benelli-m4-shotgun-black

Forearm Stippling: 110.00
https://www.tangoarms.com/products/benelli-m4-stippling

-DMW Oversized Safety: 26.65
http://www.davesmetalworks.com/m4.html

-Carriercomp 7 Round Titanium Magazine Tube/Spring/Follower: 199.00
Shipping: 10.75
https://www.shop.carriercomp.com/product.sc?productId=5&categoryId=4

-Cerakote Thin Red Line: 110.00 Estimate with shipping
-Wright Armory NP3 Plating Services: 1,100.00 Estimate with shipping
-Machining on factory Scout Light Tailcap: 40.00
-Teardown and Reassembly Cost: 300.00
-Loading Port Modifications: 250.00
-Test Ammo: 50.00
-Shipping with full value insurance: 110.00 estimate

Quoted:

Holy shit, that's funny!
And I own both.