Nearly $1.3 billion has already been spent across 35 Senate races, and the elections haven’t happened yet. The team at http://votelog.org/ have managed to tally up the total, and published them for you to follow. The site lets you follow the money back to the FEC filings so you can verify each dollar yourself. This great work was done by u/wallyrashid- if you are not following him, you should be.