I am a US citizen traveling to Hong Kong via Dubai. I have a 6-hour layover. I would like to leave the airport and return for the connecting flight from Dubai. What do I need and would I need to go through immigration again? What is the cost at immigration?
Just go to the immigration counter in the arrivals area. As a US citizen you will receive a visa on arrival for free. Roam around, get back in time. There are no other formalities you need to go through.
Since you'd already have your boarding pass for the next flight there are no check-in formalities or queues that you need to go through when you get back to the airport. When you reach airport go straight towards the departure gates and you then only have go through exit immigration and security before you are back airside. From there find your boarding gate and you're good!
Citizens of the following countries can get a 30-day visit visa on arrival, free of charge and renewable for similar period.
United States of America