Indeed that would seem more natural. Of course, this course seems set partway between Ancient Rome and the modern age, and jumps quite a bit across the two millennia. So perhaps they thought it better to only learn the word nummus.
But the internet is vast and plentiful, and even with this, it provides.
It's the ablative of price, although a little digging makes me wonder why this isn't the genitive of value.
PaulKing was asking about Roman numerals. That's a different issue.
In all of the lessons I've done, numbers are accepted in place of words as long as you're typing in English. It's only when you're typing in Latin/Spanish/German/etc. where you need to type the words, otherwise it defeats the purpose of the lesson.
Then I understand. I don't see the need for allowing numerals at all, but if that's the way it works in all other courses I can see your frustration.
Perhaps they want to keep Latin in this way... to keep a path open for handling Roman numerals (XX), and that they alone should be translated into our numerals (20). More likely: no one constructing the Latin courses ever thought about it or knew that you could use numerals.