It depends on the coin. If it were something like a common silver coin, it is basically worth the same as say a VG, If it were a 1793 cent it clearly would be much less than a VG. If something like the cent, look at auction results at a place like Heritage.
My coins tend to be common coins—large cents, Indian head cents, Mercury dimes, mostly. What I’ve been doing is figuring the AG-3 value as one-third the G-4 value. I figure they are worth at least that much. I only have two F-2 coins. I value them as one-ninth the G-4 value. I am no expert on this matter.
The rarity also plays a part. Those that are regularly available in higher grades will be worth less % at G4 or less than ones that are rarely available in fine or better.
