Depends what your looking for. Are you looking to build yourself or buy one? If you just want to play the basic 1 player side scrolling games a 60 in 1 will do. If you want the street fighters, mortal kombats you'll need something a little more powerful. There's Pandora boxes which supplies the hard drive and you can get bundles with buttons and joysticks. You can program a old computer with 1000s of games also. You want a mega, 4 player with a 50" screen? There's that also.
Check your local buy and sells. There's literally dozens in every city for sale. A quick search on mine and there's a couple priced around a $1000 loaded with 3000 or so games, trackball, etc... I paid $400 for my first one. $100 for my last one with a mame computer in a mortal kombat cab. Had 8000 games on it.