The cheapest you'll find it online is about $25, and that's rock bottom. Gohastings is a good choice ATM.
It's held its value freakishly long (almost a year now), as of today if you have a like-new copy, you can trade it into secondspin.com for $24 + 20% extra = $28.80 cash. Amazon gives $17.50 for it. There's never been a deal for it. The best deal thus far has been Amazon for $36 with a free copy of the add-on pack included.