I have set #6755, Sheriff's Lockup. What's an estimated value without the minifig?
Set 6755 is basically an older version of set 6764. There are several listings on that page missing all the minifigs - they seem to go for ~$40 (or at least those are the current asking prices).
BrickLink has a price guide that will give you both the current prices people are asking for sets as well as any details on those sets sold in the last six months.
If you can't find your specific set, you may well be able to find a similar one (either in the same theme or similarly sized) that does have the data you want.
Typically "new" or "new in box" sets will sell for substantially more than "used" sets, and licensed figures will have more of an impact on prices than generic minifigs if they are missing.