That's a real piece you've got there Tony!
It's not known exactly now many of these Spanish colonial reales from Mexico that were minted with the mint master's initials upside-down, but it can't be a lot!
The coin in our image, damaged and worn, is on sale for $129 US dollars by eBay seller Caesar's Ghost. As you can see, even with problems, these coins are very collectible. Approximate catalog values are called out below:
worn: $200 US dollars
average circulated: $400
well preserved: $1500
These are general values for all coins. Some errors are scarcer than others. It's best to take such coins to a knowledgeable collector or a professional coin dealer for an in-person appraisal.
Tony's 2 reales catalogs at around $400, or about five times that of a non-error coin. You can use this as a rough estimate for the value of your error coin. Visit our general page on these coins found here, find the value of your coin, and multiply it by five if the two initials after the denomination are upside-down.
Refer to our 'Important Terminology' page on the top left in order to properly interpret the catalog values quoted on this page.