You're asking about a quote from Shakespeare's *Julius Caesar*, but you haven't provided the full context of line 111. To understand what Cassius means by "Pluto's mine," we need to know the surrounding lines and who he's speaking to.

However, I can give you a general idea of what the phrase likely refers to:

* Pluto: In Roman mythology, Pluto is the god of the underworld.

* Mine: In this case, "mine" likely refers to a mine or a source of wealth, possibly even a metaphorical one.

Therefore, Cassius is probably suggesting that something (the thing he's referring to in the context) is as valuable as the riches found in the underworld. This could be a figurative way of saying:

* The thing is extremely valuable.

* The thing is hidden or difficult to obtain, like treasure in the underworld.

* The thing is dangerous or deadly, like venturing into the underworld.

To accurately interpret the meaning, we need the specific context of the line. Please provide the full quote or the scene from *Julius Caesar* where line 111 appears.