If you get an Xbox 360 without a hard drive can you put one in when you can afford one and how much do they cost and how do you install then

Yes, you can put on a new one. I have seen 20 gigs going on eBay for $40-80. To install, unplug all wires from the 360. On the top, you should either have a panel with holes in it, or an old hard drive. There will be a button, push it and the old hard drive should pop off. Put the new hard drive in by just popping it in, like how you took the old one off, just backwards. Hope this helps! For more specific instructions, I used eHow.com. Just type in your question.