Planchets are punched out of the coil stock and sometimes this coil stock overlaps and and when the punch occurs it creates a crescent shaped clip on the planchet. However, not all clips are perfect and or elliptical, some are ragged and there’s small clips, and multiple clips. Furthermore clips can be faked so look for the Blackely’s Effect opposite the clip, it’s not there on 100% of the authentic clips but it’s usually evident on most.
The Blakely’s Effect is a weakness on the opposite rim from the clip and is caused during the strike since the clip creates a slight inbalance of the coin while in the coining chamber. So if you see this effect then you know the clip was there before the coin was struck.
Values can range from a few dollars to several hundred dollars for clips and the larger the clip and the amount of clips can increase value, as will the denomination. Value also depends on how rare it is to find a clip for a certain date, type or denomination. So most clips for mint state and graded coins range from $50 to $350 but some can trade for more.