"It just fell off"

It can't be that expensive of a part, but I've been surprised before. Not sure how you broke it, but what is done is done. I would like to THINK the dealer would charge you just for the part and nothing to put it on seeing as it is literally a 5 second job, but I tend to doubt that will be the case. I'm growing into a skeptic the more I hear about dealer stories.
It is hard to explain in words, or pictures for that matter, exactly how to re-install the lower cover other than to say "the opposite way you took it off" but to those that know how, it is super quick and simple with zero tools so they really shouldn't charge you at all.
Can you take a photo of what is broken? It might just hold without whatever broke off - there is a lot of extraneous plastic bits under there.