Flight from Lagos to Belize throw Brazil

This wins "Confusing Post Of The Day" competition!
By "transit visa" do you mean through the USA?....if so, there is no such thing as "transit" any longer. Depending on what passport you have you will have to apply for a Visa even to change planes in the USA, or if you qualify for a Visa Waiver (check the US Immigration website) then you need to apply and pay for an ESTA online and in advance.
Qatar fly from Lagos to Sao Paulo (via Doha) for less than $500. From Brazil it is easy (but expensive) to connect to CA cities, with Panama probably being the busiest hub (you can get the bus from Panama to Belize).