It is possible to say that "a shower curtain is a shower curtain" -- and that sentiment is mostly true. The amount you spend on a new shower curtain all goes back to the level of design and materials you want.
If design and materials in a shower curtain are of no consequence to you, then a basic vinyl shower curtain will do. These are cheap -- typically selling for under $5.00.
Getting a new shower curtain on the cheap is easy to do. I would not be surprised if dollar stores sometimes have vinyl shower curtains for a buck.
The Goodwill Store sells cheap shower curtains, too -- so be sure to take a look at your local store.
It is my own personal preference not to buy a used shower curtain -- but that just speaks a little about me and my germ issues. Stuff just weirds me out -- that's all.
If you can find a new shower curtain up for bid (not "Buy It Now") on eBay, you will likely catch a nice deal there, so be sure to give that a go.
Remember, a good shower curtain is like a something, and that something is something else. I say profound things, I know!