Do you realize how many economical variables plays into that?
Besides, the worth of individual items in the game is too sporadic to conclude anything with even the most remote accuracy.
EDIT : I'd fairly guess that they are close to being the equivalent of a 5 -10 (US) dollar gold piece.
Lets look at it from the current weight of gold. A Septum is about the size of a US Quarter... roughly.
I prefer to look at basic things you can buy or services. You go up to an innkeeper and its 10 septims for a day. How much is it for a seedy motel room in some crappy town IRL? How much does a mug of ale cost in the game?, a loaf of bread? vegetables from the vendor, stuff like that. A lot of other stuff you can filter out because it will be skewed for obvious reasons. Things like a quest reward of a couple hundred gold for doing a simple task.
I missed the delivery, I'm afraid.
Considering that 1 Septum is probably closer to 8 grams of solid gold, I'd be inclined to agree.
Seriously, it's not hard math.
1 septim = 1 widget
10 septims = 10 widgets
100 septims = 100 widgets
...
...
...
1,000,000 septims - 1,000,000 widgets
i'm guessing 1 septim might be worth 10 us dollars.
We know that a septim is a gold coin, so it's a simple matter of figuring out how much a septim weighs and translating that. The only time the coins weighed anything was Daggerfall though.
The other option is trying to figure out a value of two floating currencies based on the price of commodities within both worlds. I don't think you can buy an Iron sword for ten apples on Earth though... so that kind of falls through.
That's what I was looking at earlier. Gold is $44.70 USD per gram. I would guess a Septum weighs around 8 grams. That's given a US Quarter weighs 5.6 grams.