How long is a piece of string...

The best bet is to look at eBay "sold" prices for 100-year old 18k gold pocket watches. Unless the movement and dial are exceptional, all the value is going to be in the scrap value of the gold. Is it key wound or crown wound? Is it a hunter, half-hunter or open face? Is it worn, scratched, dinged, perfect? Engraved, machine-turned, unmarked? What's the case weight without the movement.

Also check out online auction catalogues - try:

http://www.the-saleroom.com/en-gb

to see what the estimates are - but remember they're just a rough guide and may sell for a lot more.

If you can post a link to some pictures here, we may be able to help more.