If i remember correctly in chapter 2 or 3 he said he was around in his 20's. Also the mc has not been the one shopping for the group, that was done by someone else so he didn't really thought about the value of the coins. But I also agree the Mc is a little bit a air head.
I just realis you was joking about his age lol sorry
I still call bullshit. MC spent the better part of highschool and a few years after training to be a chef/line cook in his father's restaurant. Managing your ingredients, consumables, and the money to purchase it all is part of that training.
So just as a consequence of running a mercenary's kitchen for the last year or so would have taught him enough to get this done. Maybe not the conversation rates between currencies, but he should have a basic grasp of what is in his pockets.
So this farce is purely a hamfisted plot device to bring the quiet giant into the foreground so he can recieve his character development chapter.