You would have to do a "market analysis", i.e., compare it to other Leatherman Charge TTi 30 year anniversary on eBay, etc. But I don't see any on eBay, so its hard to compare them isn't it?. As to the signature, that's one of those things that may (probably) add value, but that's very subjective. The only way to really hone in on the current value is to look for the last few that have sold online. That may not be easy. Its a nice collector piece!
They were selling new for 599€ and UK was £400. Texas Tool Crafters was selling them without the xxxx/1000 print for $195 if my memory serves me right. Now the signature may probably add a bit to the value, how much, I'm not sure. Depends on the person buying it and if there are more than one buyers interested in it to drive the price up. As Tim travels the world and signs multitools on most shows he attends, probably not that much extra.
You could list it on E-bay with a high reserve let it run for a week and see what happens. Could give you a good indication of value.