Do Not Buy the 4GB Xbox 360.
It is a joke that they are selling it with such a big difference in Memory.
A 4GB console will need to upgrade to a a bigger one because that very little memory.
But i guess since you want something cheap you could buy the 4GB console then buy the 250GB memory hard drive when you need it. The Xbox 360 Slim 4GB costs $200. You could probably find it as low as $150 in some stores like gamestop or Best Buy. I doubt you will ever find an Xbox for $50.
If thats too much then maybe a "Nintendo Ds Lite" it's price has dropped to $100.
Happy Hunting
*Update*
It is pretty hard to find a console without a Hard Drive.
But here is a Used Slim for $100
http://cgi.ebay.com/Xbox-360-Slim-4-GB-Black-Console-/160600361682?pt=Video_Games&hash=item2564870ad2
