We finally bit the bullet and bought several inexpensive bookcases at a Staples, but I'm always seeing bookcases go by on Craigslist. That might be a good place to start.

I too was going to suggest Craigslist, but often they go fast so you have to watch closely. There's also thrift stores, but they usually go out the door soon after they come in.
I doubt there's any cheap way to build one unless you can find used lumber on CL. Check the materials listings. Often builders get rid of their extras that way. Be creative.
Hubby and I love to read. He shares with our son and friends, I started buying mine at thrift stores since I'm not one that has to read the latest when it first comes out. If you do that you could save enough to buy some bookselves. We have a library in our home which I thought was for books, but hubby seems to thinks it for anything he wants to cram in there. So not pretty. I did buy some inexpensive black plastic shelving at Target that I use for lighter things on the upper shelves and stack my books in baskets on the bottom shelves.
You could store them under the bed in containers made for under the bed. So much waisted space there.

storing anything under the bed is considered a no no in feng shui ;0(

thanks, i'll check it (them) out

I store my books at the village library! Generally available whenever I want to re-read. Reduces home clutter.

I frequently see nice bookshelves by decent makers on Craigslist. Check regularly if you can.