It's generally not cheap, to build a firearm chambered for a wildcat cartridge. But there are definitely different degrees of "not cheap"!
At one end of the budget scale, you find projects like the .700 Nitro Express: custom made-from-scratch brass, bullets, barrels - basically everything. Kind of interesting to read about, but something only the ultra wealthy could actually do.
At the other end of the scale are projects that maximize existing resources, and absolutely minimize the custom work. Years ago, I got together with a gunsmith and outlined a relatively high-performance 30 caliber lever action wildcat - picture the .300 Savage with a long neck (51mm case). It used off-the-shelf bullets (.30-30), brass (.308 Win), and dies (.300 Savage sizer, .308 seater). The rifle was a Marlin .35 Remington, which would be rebarreled with a factory .30-30 barrel rechambered using standard .300 Savage reamers. IIRC, the gunsmith said the only custom work might be a little "clean up" on the chamber after reaming, and maybe opening up the .35 Remington bolt face a hair (some Marlin bolts apparently come from the factory able to accept 0.473" case heads). Oh, and he could convert the rifle to a "take-down" version, if I wanted to pay for that!
Wish I could tell you how it shoots - but I never gave the gunsmith the OK to start. If you've got a wildcat project in mind that you can actually afford (whatever it looks like), I would say "go for it", or you'll likely regret it later!