Nigeria has a Visa on Arrival scheme at its port of entry which has been going on for some years now and is available to business travellers, tourists and transit visitors who were unable to obtain a visa from a Nigerian Embassy in their county of residence or where no embassy exists.
However, these visas are targeted at High Net Worth Individuals (HNWIs) and investors – according to The Nigerian Immigration website – presumably to boost incoming monies in the country.
While there are no money figures or hard to reach requests within its requirements, and the visa approval letter is free (for the time being), the eligibility criteria makes it known only a select few can use this service.

To apply, prospective visitors must complete a form and pay the associated fees for a VoA to obtain a letter which should then be presented to a custom officer at the port of entry.
More about VoA, including an FAQ session and fees can be accessed on the Nigerian Immigration Service.