It's not just costs. It's also the beurocracy (reminder: NASA is a Government Agency).
As others said, NASA had to pay the RSA (Russian Space Agency) to send Astronauts to the ISS after the Space Shuttle Fleet was retired. Obviously that wasn't sustainable longterm though some politicians wanted to either cut NASA's funding entirely or disband it entirely. NASA subcontracting private companies like SpaceX to do the heavy lifting turned out to be a viable third option.
The ESA and China are both pushing to land on the Moon soon while NASA's busy using rovers to explore Mars. As a few people said already, the reason there is renewed interest in returning to the Moon is because it will help establish the foundation for manned trips to Mars. There is also the fact the Moon has huge deposits of metals and minerals that are rare on Earth. It's a matter of national security for many countries to get a mining operation going on the Moon and...yeah.