What is the value of a 1870 Spanish peseta coin marked COPY on back side?

For starters, the Word copy should have been the tip-off: if it says COPY then it's not a genuine 1870 coin! It's what's called a replica coin, which is in effect a legal counterfeit.

Replica coins are required by law to have the Word COPY on them so people will recognize them for what they are, copies intended for display purposes but not genuine collectible coins. They're usually made of an inexpensive metal and plated with something that gives them the appearance of a silver or gold. However, they're mass produced and commonly sell for at most a few dollars in a gift shop or catalog.

You might compare them to a good print of a famous painting, or a resin cast of an historic statue. They're not intended as fakes but simply to allow us ordinary folks with ordinary incomes to enjoy the artwork in person.