Dropped letter happens when a die’s letter cavity is compacted with grease and debris and is eventually jarred loose, falling onto the planchet and then is struck into the surface of the coin. The struck through a dropped letter will create and incuse of the letter and it must be the same size and font style found on the coin is struck into.
The dropped letter error in the image above was graded by PCGS MS65 RD and it sold for $161 but a Morgan Dollar with an O dropped letter sold for $8,518.75. So there’s a wide range of values on these and I would be cautious on buying raw examples since many collectors seem to see numbers out of any shape and declare them mint errors.
Also see strike through