' a major investment' or 'a considerable investment' might be better alternatives than 'important '.
You are right.
In French, (I don't know why) adjective "important" is massively used to the detriment of many other meaningful and relevant synonyms: énorme, considérable, majeur, immense...
The French "important" can be translated to English as: considerable, substantial, large, sizable, significant, and...oh, yes....important. Actually, ANY of these terms would make perfect sense in this sentence. It seems like important = "important" is just too easy to use when one is in a hurry. In the meantime, learners are never exposed to these other valid choices.
Unfortunately, while Duo offers "big investment" as a correct answer, it doesn't accept "large investment.'
When translating it to "It's a considerable investment" I got it wrong and Duo suggested "sizable" instead. I didn't report it when I saw it, but now that I see that 'considerable' should have been accepted, it's too late for me to send a report until it appears again. Therefore, I'll just leave the comment here for now.