A full cage with us will run about 4k at the minimum. We consult with each customer for class rules and setup before building as well. Our fabricators also hand fit and notch each tube very precisely. Baseplates are custom made and Mig welded to the body. The rest are fully tig welded by hand. The cage in my car would probably cost a customer about 4200. It's very light, very stiff, and very nice. It fits the chassis super tight in critical areas, with some models the tubes are so close to the unibody we can mig weld them straight to the chassis without gussets. A cage that is integrated properly to the chassis is much much stiffer. Custom craftsmanship just costs money.
Most cages below this cost will all use the same diameter and thickness tubing. Skip features like foot protection, dash bars, head protection, bracing. And are mig welded to save time on labor.
The shop recently did a full cage in a car and retained the air conditioning and vents in the dash while still fitting a dash bar... Weird build.