That's a little bit like asking "how big is a rock".
This should help you make a decision.
First thing to do is gain an understanding of what the markings mean for gold filled items. When you understand the markings, come to terms with the idea that gold filled items tend to be worn from use----so the basic math will require a correction---to compensate for wear that may not be easy to predict. In that regard, look for areas where wear has penetrated the thin gold alloy layer---such as corners. It's easy to test with nitric to insure that you are seeing base metal, not gold. Calculate how much you'd like to pay for the assumed gold content, then make your offer. Hope like hell you get it right.
My attitude towards gold filled items was that I wanted to get them cheap, or not at all. For example, I had a top limit I'd pay for eye glasses, and it was low. 25¢ was the most I was willing to pay, and hauled them home in fair numbers as cheaply as 10¢ per pair---and that's when they were 1/10-12GF. Gold, then, was in the $300 range. On the average, they were worth over $1.00 per pair.
Harold