This article by By Katherine Huggins is published by open secrets. Here is an excerpt:

More than $560 million went into the 34 upcoming Senate races nationwide in 2021, as Republicans fight to gain control of Congress and Democrats strive to increase their 50-50 marginal majority in the Senate.

That’s up from $327 million raised by Senate candidates in 2019, the first year of the 2020 presidential cycle. In 2017, the first year of the 2018 midterm election cycle, Senate campaigns brought in $311 million and raised just $211 million in 2015. 

Of the 34 Senate races, 20 raked in more than $10 million dollars in campaign contributions during 2021, with much of the money focused on battleground elections.

Continue reading here. Also, take a look at the transparency reference websites Judgepedia and Ballotpedia that are also great resources.