Update: I paid $60 for a museum replica coin and it turned out to be a cast copy of a known fake
I picked up a small Roman coin replica at a gift shop in Bath last spring for $60, thinking it was just a nice souvenir. Then I read up on how to spot cast copies and saw the tiny seam around the edge and the soapy looking surface, which are classic signs of a mold made from an existing fake. The museum staff were honest when I asked and said yes, that is a common replica copy, and the real thing would be in a case. Has anyone else bought a souvenir coin and later checked whether it was cast from a known forgery?
I read a blog post from a coin collector who said museum gift shops often sell replicas made from molds of old forgeries, so the copy is a copy of a fake. The soapy surface you saw comes from the mold, not the metal itself. It is a known problem with cheap souvenir pieces, and staff usually do not know the history behind them.