The Brazilian government’s agency responsible for fuel, the Agência Nacional do Petróleo, Gás Natural e Biocombustíveis better known as ANP, has a useful tool on its website where you can look up the average prices for fuel all throughout the Brazilian territory: Levantamento de preços.
[As usual on the websites of a government that calls itself champion of free software, the page does not work in Firefox]
It is quite simple to use. You need to indicate the period of time (Selecione o mês, usually the current month), the geographical area (Brasil, Regiões, Estados, Municipios) and the type of fuel (gasolina [petrol/gas], álcool [ethanol], diésel, LNG or LGP). Click on Processar to get your results.
The results page is simpler than it looks. The column that interests you is the one with the title Preço médio (average price).