We already knew that. Each year, members of Congress have to report their economic interests, data that's made public and compiled by the Center for Responsive Politics. That data doesn't provide a hard number; it provides a range. (That's because the report itself asks for a range of values.)
At the top end of the spectrum are three Democrats, Mark Warner, Dianne Feinstein and Richard Blumenthal. (The most recent data is for the year 2014.) The 25th-richest member is Jeff Sessions of Alabama, whose average net worth was $7.5 million.
That's not the ballfield where Bernie Sanders plays. He's 81st on the list -- meaning he's the 19th poorest member of the body. In the graph above, you'll notice a dashed line just to the right of the zero marker. That dashed line is the $2 million mark. Only one of the poorest members of the Senate is anywhere close to that -- only because she has a very wide range of possible net worths.
Sanders is not a millionaire. On economic issues, he walks the walk. The only weird thing about his statement about his relative poverty during the debate -- Sanders is still worth more than ten times the actual poverty level -- is that he he dubbed his lower net worth as "unfortunate."