Visa fees depend on a number of things: your nationality, the amount of time you are going to spend in the country, the number of times you are planning to enter the country and the type of the visa you need.
Generally embassies charge more for visas which allow you more than one entry and let you stay in the country longer. Also business visas are often more expensive than the tourist ones.
For instance, a 30 days single entry visa for Vietnam is significantly cheaper than a 30 days multiple entry visa, or a 90 days visa.
Visa fees are always regulated by the embassies and governments of the countries and are always reciprocal. That means that if the US charges $160 for a tourist visa, majority of embassies will charge US citizens the same amount if they apply for a tourist visa for that country.
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