The yellow fever international vaccination certificate is required both for the visa and for entry in the country. There is a counter at the airport where one has to show the certificate. In the absence of such certificate, you can bribe the agent for a few thousands of CFA.
A consular letter of your bank stating how much money you have is also required for obtaining the visa. NOTE: a bank monthly statement is not enough. It has to be signed by your bank.
It costs around 150$ for a 6-month visa, if I remember well. But you can also get a 3-month visa for half the price.
Of course, you need 2 photographs and the form filled. You also need a confirmed retrun ticket in order to apply for your visa.
To exit the country, it costs 10,000 CFA in airport taxes. If you do not have a valid visa, then it costs 50,000 CFA.