This is jargon, rather than the mainstream word in a given language, but:
In the parlance of tipped employees in various service industries (hotels, restaurants, casinos, etc.), one hundred dollars is referred to as a bill. However, it’s use is restricted to money. You can say “I made three bills tonight” or “that jacket cost two bills”, but not “How many people were there? About a bill.”
Compare the use of grand to mean “one thousand dollars”.
(Bolding mine)
This may be a nitpick, but counting in Hindi after twenty goes something like this:
twenty-one
twenty-two
.
.
.
twenty-eight
one-less-than-thirty
thirty
And so on. Strictly speaking, we’re neither adding nor multiplying here, but it may be splitting hairs finer than you want to. 
Well, it’s worth noting, anyway.
While I would love for someone to pull (against all likelihood) an etymologically distinct word for 13 out of somewhere, it’s these little things that keep the thread alive.
And the subtraction-names are interesting & fit the thread well enough.
Good point, and some surfing starting with wiki page on positional numeral systems gives some great examples. Especially the page on vigesimal gives a lot of words for ‘twenty’: vingt in French,* tyve* in Danish, ugain in Welsh, fiche in Gealic and of course score and some others.
Other higlights:
and
(I’m not sure which languages are considered Indo-European, so I went with the any will do part of that request)
As for numbers greater than 99 in the game of darts ton is used for 100 (well in Dutch anyway). In Dutch, ton can also mean 1000 or 100 000 (related by the the fact that a ton (a barrel) could fit 1000 kg worth of 10g silver coins)
Tyve has the same “two-times-ten” etymological roots as twenty.
Sounds interesting nonetheless. What unusual number?
Does slang count? I’ve worked around survey crews in various parts of the country and “punk” is usually used instead of “eleven”. This is to avoid confusion with “seven” in situations when it is hard to hear.
Slang, but would “baker’s dozen” qualify? Stuff like a ream=500 sheets doesn’t qualify as under 100, but that type of number—used for very specific things, like paper or doughnuts—is where I’d think you might find unusual words for numbers.
Pair/brace for two, but that’s under 10.