Many areas of Nepal either come under restricted regions or conservational regions. Remote rural areas, particularly close to the Tibet border, require a restricted area permit from the government of Nepal. This applies to those trekking in the Manaslu, Mustang, Nar-Phu, Tsum Valley, Dolpo, Humla, and Kanchenjunga regions. 

Permits range from US$10 to US$500 per week, and you will need the help of a registered trekking agency to secure one. Similarly, you have to get permits to enter another region. Plus, you have to take tickets that come in a few bucks to enter monuments. For more details, visit the Nepal tourism board site.