The standard answer is whatever somebody will pay you for it. These are collectible guns. There is a market for most grades in good condition. The best way to get a reasonable answer for your particular gun is by an inquiry to: email@example.com
However before you jump there you need to have some basic information ready and photos will vastly increase the accuracy of any assessment the well informed LACA members will give. Instructions on how to post photos are on the forum site.
For the value of a Lefever the two most important issues are Grade and condition. Most, but not all, Lefever Arms Co. Guns have a Grade assigned including the later DM Lefever Guns. We will not get into early Lefever hammer guns here because they are so rare. The Grades will be discussed below.
Other issues are alterations. Sometimes these can be hard to detect. The most frequent and costly in terms of values are if the barrels have been cut. Almost all Lefever gun barrels are exactly 28, 30 or 32 inches in length. A few are 26 especially in smaller guages The length is measured from the breech end of the barrels themselves, not the rib extension, to the muzzle end. Another clue is to look at the barrels end on. If the sides of the barrels do not touch or come within a tiny fraction of touching they may have been cut.
Stock alterations can be very difficult to detect if done well and may not detract much from the value. Poor stock alterations or broken stocks obviously reduce the value.
Case colors on the frame (case colors are the various shades of blue and purple produced when the frame is originally hardened at the factory, or can be done later as a part of a restoration) are a positive factor, especially if original. However, original case colors on a gun that is an old as a Lefever that was used in the field are rather rare. Mostly they are found on high grade guns that were lovingly kept and lightly used, or sent in for restorative work. Sometimes case colors are faked by applying an acetylene torch to the frame. Obviously, this is not good.
Barrel condition is important, especially if the buyer intends to shoot it. Dents in the barrel can usually be repaired if not severe. Minor pitting inside the barrel can sometimes be overcome. Lefever guns came with either steel barrels or one of several kinds of twist steel. You can tell twist or Damascus barrels by the pattern on the barrels that spirals the length of the barrels.
Condition can be difficult for an amateur to describe. Rust is bad but doesn’t necessarily make it worthless. Dents and pitting in the barrels (look down the length of the barrels after running a cleaning brush and cloth) certainly detracts from value, but again doesn’t make it worthless as some of this can be repaired.