@opYou have no idea. That's why you even bothered asking. Nothing against you, but once you find out, most guys realize steel is more than they want to get into.
I don't have a forge, but I do have a basic machine shop. Fabricating metal is a new dimension in expense. With the forge you'll also want an kiln for tempering, at least a drill press and mill for machining, maybe a lathe, lots of fixtures like anvils and vises, a power hammer, band saw, quenching equipment, a free-standing belt/disk sander, probably an arc welder and oxy-acetylene torch, a ventilation system, compressed air, a heavy-duty metal shop vac, plumbing fixtures, sewer, steel bar stock, steel racks and shelving, maybe a sheet-metal break/shear, spot welder, a few hand tools--tongs, hammers, magic potions--and some not so magical--to use in heat treating/hardening, lubrication and adhesion, possibly liquid nitrogen/cryogenics, climate control, personal protection equipment (not a big expense, but absolutely essential), precision thermocouples/rtds, countdown timers, benches, grinders (bench and handheld), cutting wheels, and the enclosed space to hold it all.
You can move up from those basics. On the used market we're talking $17,000. I don't forge anything, just basic machining of small to medium-sized pieces. Just the vise for the drill press cost me $600. Tooling also gets expensive, but hobby guys can usually buy each tool once. You won't wear out mills, cutters and bits. But they can break.