Translation:Their research costs three billion rupiahs.
I've never heard "rupiahs" as a plural in English, I think we just say "rupiah"
Don't you also say dollars in English? It's just like saying three billion dollars, three billion rupees, three billion pesos. But in Indonesian, of course you don't add the 's'.
I get that it makes sense to pluralise, I just don't think I've ever heard it used that way. It's pretty common not to pluralise loan words from languages that don't. "Pokemon" is the first one that comes to mind, though not the best example.