I've built a number of bumpers.
Depending on what thickness you use, and if you use tube (and what type of tube), you are probably looking a 1-200 bucks in steel. Add in another 50 for hardware (probably will be less, but its good to over budget). Lastly, add up consumables. If you don't have a large press, or serious heating equipment, you'll likely be doing a lot of cutting to make the bends (assuming an ARB style). That much cutting will necessitate lots of welding, and, if you want it to look good, lots of grinding. Cutting disks, flap wheels, grinding wheels, welding gas, welding wire, etc... all cost money.
My rear bumper cost 200 in steel, plus 200 in hardware/consumables, plus a 100 for powdercoat.
My front bumper cost 100 in steel, 25 in DOM, and 50-75 in consumables/hardware.
My sliders were 100 in steel, 75 in DOM and about 25-30 in hardware/consumables.
Lastly, be prepared for it to cost more than you think. I've NEVER had a project come in on budget.
Baby Beast- Mini Truck in progress