That's a tough one.... have you previously sold it? If so, same price again. I resell everything at the same as the previous price.
If no then what is the clip worth? How long are they going to use it for? Are you selling them FULL rights or single/multiple use rights? Lots of variables to take into account.
I sold a clip that I took for my b-roll.... it was about 25sec of flight. I was flying in reverse and I "almost" clipped a church steeple (totally didn't see it until I reviewed the footage). This would be a TOUGH clip to replicate and I had someone who wanted it VERY badly. I sold it to them for their own use ONLY but for lifetime use, I kept full rights of the clip, and they can't resell it.... I sold it for $1,500 but keep in mind it was a "Technically difficult" clip to remake. I wouldn't think most short clips are worth anything near that amount.