Shehu Sani, a Senator for the governing All Progressives Congress [APC] party, said there is little accountability about how the money is spent.
He has called for the payments to be scrapped, saying they are a key reason why people choose to enter politics.
“I decided to bust it open. It was a moral issue,” Sani told the BBC.
“The National Assembly is one of the most non-transparent organs of government. It pricked my conscience and I decided to burst the bubble and open the National Assembly to public scrutiny.
“If the expenses payment system was ended then Parliament would only be attractive to people who contribute ideas.”
The money is paid on top of a Senator’s salary which is more than $2,000 a month. They also receive a yearly constituency budget of $5.5 million.
Nigeria’s minimum wage, meanwhile, is just $50 a month.
The Senate confirmed Sani’s revelation but described it as “nothing new”.
“Almost all holders of elective and appointive offices have running costs allocated to their offices that cannot be said to be part of their salaries,” a spokesman said.
The National Assembly deliberately made it impossible for Nigerians to learn exactly how much politicians were paid.
It took the outspoken Sani to lift the lid from the inside. The salaries, at around $2,000 a month, are relatively modest. But via the back door Nigeria’s senators are receiving some astonishing expense payments for which there is little accountability.
After serving two terms, a Senator’s expense payments total an eye-watering $3.5 million.
You get a clue as to how some of the money is spent when you visit a Senator’s office. There will always be a queue of people waiting to have a quiet word – often expecting a hand out from the Senator’s deep pockets.
But how much stays in the pockets and how much is paid out – no one knows.
Civil society groups in Nigeria welcomed Sani’s decision to reveal how much Senators were receiving each month.
Usman Abdul, president of the Campaign for Democracy, criticised the payments.
“You can take good care of over 100 graduates from a Senator’s salary,” he told The Punch newspaper. “The legislature should deliberate on the abject poverty and unemployment in the country.”
Meanwhile, Nigerians – who, up until now, had little hope of discovering the financial arrangements of their MPs – vented their anger on social media.
“Now we know where all Nigeria’s money goes to,” one person wrote.