I ended up practically tying myself in knots working this out for Reasons. This is pretty much the conclusion I came to, and a copy of something I posted elsewhere, for those reasons. I hope you find it helpful; if people would like, I could copy/paste this whole thing into an actual post somewhere to make it easier to find down the line...

Back to the topic at hand. Roman currency:

4 quandrantes = 1 as (both bronze coins)
2½ asses = 1 sestertius (a silver coin - well, 'silver') (also known by the Anglicised version 'sesterce'; they're both correct.)
4 sestertii = 1 denarius ('silver' - roughly a day's pay in the early 1st century AD)
25 denarii = 1 aureus ('gold')

Modern currency is no longer based on the gold standard, which really complicates things. BUT. 1000 sestertii = 4oz gold, so working from the modern value of gold, hopefully, we can get this.

In today's money, 1oz gold would cost between £930 and £940. To make things REALLY easy, I'm going to round up by quite  bit and say 1oz gold costs £1000.

So. 1oz gold = 250 sesterces. £1000 = 250sesterces. 1 sestertius = £4.

I'm not sure I can get a closer equivalent without bending space and time. For ease of arithmetic (and because it'll make it a damn sight easier) I'm going to say 1 sestertius = approximately £5 or US $6.5 (modern conversion is SO MUCH EASIER!)

These figures give a very rough rule of thumb! Modern currency has denominations of notes and coin much closer in value to each other than the Roman currency where the value escalated sharply between a denarius and the most valuable coin of all, a gold aureus.  This is the currency system as used in the time of Augustus; it did change as the Empire progressed, but I think this is the system most people are familiar with. Why is all this so complicated? It was complicated even back in the day! Roman currency.