score:2
I answer myself. After some extensive researching, I've found this blog post in Spanish that says:
The visitor center has some confortable huts with beds, bathroom and AC [..] is the only place you can stay overnight, because you cannot camp.
Also, it explains that there are crocodiles in the area, so it's not a good idea to camp in the beach even if it's allowed. They don't explain if the fee is payable once or you have to pay it every day, but I suppose you have to pay only once because this post explains every option with a good budget detail and don't talks about paying a daily fee. The bad part with the post is that prices are outdated, so take these prices as reference, but expect to pay a bit more.