When a US citizen travels to Nicaragua, what does it cost to enter the country? How long can the stay be without having a visa or paying a fine? Is there a resource online where up-to-date answers to these questions can be found?
According to the Bureau of Consular Affairs:
A visa is not required for U.S. citizens; however, a tourist card must be purchased for $10 upon arrival. Tourist cards are typically issued for 30 to 90 days.
You can see all pertinent information here. It is accurate.
You don't need to have a visa to visit Nicaragua. The only request is that you buy a tourist card for $10 and have a valid passport.
You can find more detailed information in this link, it is pretty acquire. The US embassy has good service for Americans in this country and good Facebook page.