In their big argument, Cassius is talking about how noble and loyal he is. Brutus has just said to him - in anger - that Brutus sent to Cassius for certain sums of money which Cassius denied to give him. Cassius is vehemently disagreeing: he hasn't denied Brutus any money, he says! And, at the height of this disagreement, this is what Cassius says:
There is my dagger,
And here my naked breast; within, a heart
Dearer than Pluto's mine, richer than gold.
If that thou best a Roman, take it forth;
I, that denied thee gold, will give my heart.
Cassius presents Brutus with his dagger, strips his shirt off, and tells Brutus to cut out his heart, if indeed Brutus is a true Roman. Instead of gold, Cassius will give up his life, he says, rather dramatically. Pluto was the Roman god of certain metals (the Roman word "wealth" derives from his name) and Cassius is basically saying that his heart is extremely valuable. It is worth more than all the precious metal in Pluto's mine (most of Pluto's metals were mined from the ground) and richer than gold. And he'll still give it up to Brutus - to prove to him that he didn't deny him money.
Hope it helps!