weigh what you can get against what you would have to spend.
You'll have to spend more than 100 in MDF to make a cab the size of a TMNT. But you may be able to get other gutted, or junk cabs from local clubs or routers for less.
Also, how important is crispy new and clean vs. original vs. labour savings, vs. time savings vs. freedom of design vs. your handiness factor...
There's no one equation that fits everyone. Your cheapest functional cabinet IMO will be to buy a crappy working title locally for about 200-300. Including a monitor, some controls, and coin door. Then budget 100 or so for replacemnt controls and interfaces like the ipac or arcade/vga cards and another 50-200 for a trackball if you want one and can fit one in. Paint, graphics, tools....
You're looking at 200-500 cheapest way with results varying by available raw materials, time invested, and scavenging ability.
Join a local arcade collector's club. There's one where I live and guys are often giving away stripped cabs, parts, or selling them cheaper than you can get them off ebay.