Wet Ink Transfer.

Well... your coin/note is worth nothing. It's only worth as much as someone's willing to pay for it.

And especially for errors, that could range so much because the market is so small. You can get the Guide to Small Size Paper Money (it has an errors section) but then again it's just a reference.

Are you planning to sell it? I personally save all the errors I have... well, because they're errors and there's only oh-so-many of them out there.

It is as WD eluded to Adam....Hypothetically, if you have it up for sale, and WD and I are your only interested buyers, I think it is worth $3 to me, and WD thinks it is worth $40 to him, then what does that tell you about the value of the note?

Many error notes do not have assigned values, because interest level is low, prospective buyers are few, and so a seller takes what he can get at a particular point in time.

The dealer that you mention may have had a $25 price tag on that note last week, it didn't sell, he puts a $100 price tag on it today, and tomorrow it sells for $100 to someone who didn't see it last week for $25.
Precise, or even approximate value is nearly an impossible assessment on some notes.