Symbolism, Imagery, Allegory
This one gives theater prop departments everywhere a run for their money (silly pun intended). Seriously, we dare you to count the number of times coins are exchanged in the play, because it seems like everyone is always passing around the scrilla. So, what's up with that? Well, let's think about this in terms of who has money, who hands out money, who loses or doesn't have money.
The first time we see coinage circulating on the stage is when Viola gives the sea captain a few coins for cheering her up about her (possibly dead) brother in Act 1. For Viola, who also gives money to Feste and poor Antonio as he's carted off to jail, money is a symbol of her generous spirit.
For someone like Sir Andrew Aguecheek, who burns through money like there's no tomorrow, excessive spending and the wasting of ducats is a symbol of his foolishness and excess. (Come on. The guy's got to write home for more money before we're halfway through the play.)
For Antonio, who gives Sebastian his purse (don't laugh, every guy had one back then), money seems to represent Antonio's willingness to give himself (in friendship, love, etc.) to Sebastian, who thinks nothing of taking it but gives nothing back in return. This is made even more apparent when Antonio is carted off to jail and desperately needs his cash to buy his way out of the jam. Meanwhile, Sebastian is off spending Antonio's money and hooking up with Olivia.
We know what you're thinking: what about Feste? Someone is always giving that guy money in the play. You're right. Feste is a professional performer who works hard for his money. (Unlike Sir Toby, who sponges off everyone else.) Feste's really clever (a bit of a con man, actually) when it comes to getting people to empty their pockets for a few jokes. At other times, Feste is paid to run errands, which reduces him to the status of a menial servant. In these moments, the exchange of coins between Feste and the upper-class characters seems to highlight the class difference between "lowly" entertainers and those with power and wealth.