It can be a little bit hard to estimate because these types of builds are a great way to dispose of otherwise throwaway computer gear. You don't need to buy a new PC to run emulators, so whatever was decommissioned after your last upgrade is perfect. Same deal with displays, personally I think an arcade cabinate is a great way to use an an old 20"+ CRT monitor or TV, it looks more authentic and IMO better than using scaling/oversampling algorithms and an LCD display, but I understand people have weight/packaging/convenience requirements.
I built a full cabinet with some guys at work a few years ago (not as outrageous as it sounds as we were a handheld/mobile games developer) and would estimate the build cost a couple hundred bucks. We did scrounge around in our discard pile for the PC we used, a monitor, and one of the guys had enough woodworking experience to do the cabinet construction so we were only in for the cost of raw materials on that, basically a few sheets of MDF and screws. I can't remember what we covered it with, but the guy who did the woodworking laminated it himself just like the real deal, if memory serves you just buy that stuff by the sheet and apply with contact cement or spray adhesive.
The control hardware itself is surprisingly inexpensive. For the control board I used an Ultimarc iPac4:
For $65 this lets you wire up enough buttons and joysticks for a giant 4-person control panel. For $39 you can get the iPac2 and that's good for 2 Street Fighter style stick/button layouts and a few auxiliary buttons. It plugs into a PC and pretends it's a keyboard, and then you can map the buttons on your layout to keys for MAME, and Ultimarc has their own programming application that is pretty good.
Most of our controls were Suzo Happ:
You can Google and find a few vendors that sell small quantities to the DIY cabinet market. Expect buttons to run $1-2 and joysticks $10-20. We ended up getting really into it and buying joysticks from a couple different brands and optical joysticks and other gimmicky things, but in the end the Happ competition buttons and competition joysticks were the overall favorites, which also happen to be among the least expensive. If you want to try other brands, especially Japanese imports, it might be 2 or 3 times as much per item. In general everyone seemed to feel like the more basic the stuff is the better, although a stick with a restricted 4-way gate can be really nice if you have dedicated Pac-Man players or any other games that didn't use an 8-way stick, it can actually be awkward when you are inadvertently on one of the diagonals that don't work in a given game.
The unconventional controls are significantly more expensive, like if you want light guns, trackball, spinner, steering wheels. We played with some of that stuff and it was fun but IMO, unless your favorite game ever is Arkanoid or Ivan 'Ironman' Stewart's Super Off Road, it's really not worth the hassle. Just build a panel with 2-4 Street Fighter style layouts and if you want the other stuff maybe build another cabinet or separate control boxes you can plug in.
I just did some quick googling and it looks like there is quite a bit more stuff available than there was when we built, so have fun.
Wiring the stuff up is just a couple of spools of solid copper wire and crimp connectors, the stuff comes in big bags for a few dollars at electronics supply stores. If you're doing a big panel, it's definitely worth investing in a pair of decent crimpers with interchangeable dies, it's basically impossible to consistently and efficiently work with the $10 stripper/"crimper" combo tools. The wiring is not sophisticated at all, it was basically my first "electronics" project and it's really just one signal wire from the iPac to each button/stick direction and then a common ground wire, easy but tedious work.
Tools wise, it's saws, drill, crimpers, screws, maybe some glue. If you have to buy that stuff it would increase the cost of the build but it's pretty basic stuff. I didn't do the full cabinet build but I did a prototype control panel myself that was just a freestanding 3-player panel built of three pieces of MDF screwed together at right angles like a basic shelf. Basically just had to drill out all the holes, I think button and joystick holes were all the same size, a few screw holes for securing the joysticks, the buttons have screw-on retainers on the back.
So basically I would say you start at low hundreds for an iPac, controls and raw materials, and depending on how much of the build you can do yourself and how many tools you have to buy it could vary quite a bit. No spare PC? A few hundred bucks? No spare monitor? A couple hundred? Need to buy all the tools? Maybe $100-200? Don't want to build the cabinet yourself? Kits are a couple hundred bucks.
But I think anyone on this forum is resourceful enough to beg and borrow tools and expertise and spare parts and put something together pretty cheap.