I don't.
Or rather, if the situation warrants it, I don't make them pay in coins. I make them pay in favours, services, oaths of fealty, or similar.
I usually handle money in a very abstract way in my campaigns. The characters are not 'adventurers in search of fortune', they have the means to support themselves in a lifestyle appropriate to their station. In all but one campaign, at least one of the characters was from a *very* wealthy family (we are talking Sel-Kai merchant lord or Rhakhaan Duke level of wealth and influence), so money is usually not a concern.
But in many cases, money is also useless - either useless because those you want to deal with do not use it for exchanges, or useless because those you want to deal with already have more than they could ever need. But favours or services ? That's an all-time favourite for a good reason: it's universal, the characters can always pay (they might not *want* to, but they can), and it's something you can actually use contextually - it makes the NPCs needs, choices and expectations matter.
If money really is the thing that can make the deal, I have them pay and just say something along the lines of "this is going to make a dent in your finances, and your father will likely have something to say about it". Basically, it means "it costs money, but the real cost for you will be in another coin".