Members of Congress make less than $ 200,000 per year but are allowed to make additional income if it is non-salary income from their investments, corporate dividends or profits. Also important to note, member of Congress are strictly prohibited from earning or accepting income that may appear to be intended to influence the way they vote on legislation.
But of course there are loopholes.
One loophole is enriching family members either directly, such as paying them with campaign funds, or indirectly such as creating 2nd or 3rd tier companies that benefit from legislation a la Harry Reid.
Then you have thinly disguised money laundering schemes. A member of Congress may push legislation that favors a particular group who just happens to later pour in megabucks in the form of campaign contributions to that particular member. There are many variations of this scheme but the overall approach falls under the heading of The Tony Soprano Approach.