I would look at:
1) Any good traffic?
2) SE results / term popularity / Saturation
3) Need / Overture / Demand
4) Other extensions reg'd?
5) Potential for long term resale ROI and/or click rev. Is it a limited name such as a LLLL.com? Or perhaps an upcoming tech name, bio, medical, gaming, gambling, porn?
6) Brandability, Uniqueness, Standoutability
7) Pesonal Interest, sentimental value.
Those are usually the questions I ask myself and research before makeing a decision to let something drop or sell it at a low cost.
Lay them out on an Excel sheet, find the values and rate the personal ones from a 1-10 then sort it all out.. Let the numbers tell you what to keep. Or reg, that's what I do.
Hope this helps you,
GG