Itx combo setup for around $400 and a $100 eBay special screen. Heck, I just sold one of my Itx as a full working desktop for $100. Look around for deals.
Of course, if you want to play a dvd, you need a dvd drive
and if you want to control it you need a keyboard, and or a touchscreen
and if you want to see it clearly you need a good screen ($250+)
and if you want a good power solution, you need an opus
If you build it cheap, you will do it twice, i guarantee it. You will quickly outgrow a cheap stsyem, and you probably wont like it to begin with. Do it right and take your time, it will be worth it in the end. While your saving up... and some more! There is really a TON of info here...you need to sort thru it and make decisions.