Mr Sanders' annual income, which is shared with his wife, is less than his multimillionaire rival made for three recent speeches delivered to Goldman Sachs employees.
The banking giant paid Ms Clinton $675,000 (£475,000) for the appearances. She and her husband have an estimated net worth of $110m (£77m), far surpassing the Sanders, who are worth around $300,000 (£210,000).
Income inequality has been a key plank of Mr Sanders' campaign, leading Ms Clinton to challenge him to publish his earnings. On Thursday, she said: "I've released 30 years of tax returns, and I think every candidate, including Senator Sanders and Donald Trump, should do the same."
The veteran Senator rapidly fired back, saying: “They are very boring tax returns. No big money from speeches, no major investments.Unfortunately, I remain one of the poorer members of the United States Senate.”
Ms Clinton generally commands a minimum of $225,000 per speech, or over four times as much as the $52,000 (£36,000) the average American citizen earns in a year. She once pocketed $315,000 for a 20-minute address to Ebay staff.
Mr Sanders' income, by way of contrast, is largely derived from his $175,000 Senate salary. His wife, Jane Sanders, makes $5,000 annually as a radioactive waste disposal commisioner, and the couple also receive social security benefits and a small annual pension.
With an estimated wealth of around $3m, the average US Senator is worth 10 times as much as Mr Sanders. While his income puts him in the top 5 per cent of US earners, the Vermont senator is one of the poorest 20 per cent of members of the Senate.
He also has tens of thousands of dollars in credit card debt, a rental property in Vermont capital Burlington and a condo in Washington, DC.
Mr Sanders has raised nearly $140m to support his campaign for the Democratic presidential nomination, without recourse to big business or Super PAC backing. His campaign is almost entirely funded by an estimated three million small donors.
All 10 companies cited by Mr Sanders as "America's biggest tax dodgers" have donated directly to Ms Clinton's $220m campaign or the Clinton Family Foundation.
Neither Mr Sanders nor Mr Clinton, however, come close to matching Donald Trump. The tycoon has not published any tax returns, but is believed to be worth somewhere between $3bn and $10bn.