If you've been thinking about building a custom bike, it's a great idea to start with a free bike. That way if you get into it an decide it's not for you, you haven't lost a lot of money. Just make sure you get a title before you put any effort into the bike.
Here's my free '86 Honda Rebel 450 (Before):
After - This was my first custom bike build. It took years. I sold it for $2200 and probably had less than $1000 in parts into it.
Here's a $400 bike I got from the pawn shop:
This build took a little less than a year. It's worth a more now than a mint version of the stock bike would be worth.
Here's a $400 '75 CB400F that I bought last fall:
Here's what it looked like a month later (I've had a lot of bike building experience since that Rebel). I sold this one for $2500.