The finance ministry said in a statement that its formula now projected an average price of $79 per barrel next year, below its proposal of $82 made in September and an $81 price approved by lower house lawmakers this month.
Finance officials provided the new estimate to a meeting of lawmakers, including senators who are currently debating the 2015 budget bill, the ministry said.
International oil prices have slumped because of a production glut and concerns about slowing global growth. Heavy Mexican crude is cheaper than global benchmarks and its price hit a four-year low this month.
Latin America's No. 2 economy is a top crude exporter to the United States and Mexico's government relies on oil sales from state-run Pemex to fund about one-third of its budget.
Earlier this month Mexican lower house lawmakers approved budget legislation for 2015 that lowered the estimated oil price while factoring in a weaker exchange rate for the peso. A weaker exchange rate means the government can count on more pesos per dollar of crude exports.