@Serelanordir
Well, the coins back then were made to have the value of the material they were smelted with, so a copper coin had the value of actual copper based on the coins's weight (this also means the whole nonsense most of these isekai have about 100 copper = 1 silver is bs). At the same time, the purity of the coins didn't reach 80% in most cases, usually hovering between 60 and 80%, though that depends on the element they were made with (some elements are easier to isolate than others, the purification techniques back then favored certain materials, etc.).
Taking all that into account, 1 gold coin, with a weight of about 30g (remember gold is quite dense), would have at most 24g of gold.
Let's take 18K gold as a base, which currently has a value of 29.54$/g (18K = 75% purity gold, or 18 parts of pure gold every 24 parts). A single gold coin of 30g would be worth 886.2$.
Kaoru is trying to get 80,000 coins, and I believe she had 40,000 before hiring the mercenaries, meaning right now she has 40,000 x 30 x 29.54 = 35,448,000$
TBH, it's not much for the military (a single tank can cost a big chunk of that), but for a small mercenary group maybe it's enough, moreso for a single day of using a few rounds.