Broken iPhone 4--How Much It Worth

A friend of mine recently cracked the back glass on an iPhone 4.  It worked fine, but someone else tried to replace the the back of the phone, and things got bad.  We weren't able to put it back together properly, and it basically went dead.  It's now missing screws and won't turn on, but I'm confident many of it's components are still functional.

How much do you think it's worth?  I'm checking eBay prices, etc. but I'm sure someone in this forum has a few suggestions or advice.  I assume I can sell it part-by-part, or as-is -- which strategy will get me more money out of it's sale?