Just as an example.
A bonds weld in kit for a GTR will set you back around $550. That will get you a hoop, single diagonal, rear legs, harness bar and mounting plates.
It will take about 6-7 hours to fit properly, including cleaning the floor, shaping/ welding plates, knotching each tube joint,TIG welding etc etc
So on average $1100 fitted give or take if it's TIG welded.
A weld in rear half. A really nice specific bent hoop (max 5mm gap to pillar), single hoop diagonal, rear legs, full X rear section, harness bar, leg supports and base plates etc etc.
It takes about 10-12hours to do And costs $1450-1550
Or get your local exhaust shop to knock up something trashy for 800 bucks.
It's really important to shop around, there are heaps of cage builders out there who do just as good if not better work than the known guys and are willing to do more for less and spend the time with you to give you what you want as opposed to getting what your given.