The Brennan Center has conducted this analysis, concluding that at least $1.9 billion was spent on online ads in the 2024 election.

From the report:

Political advertisers spent $1.9 billion on online ads for the 2024 election on the four largest digital platforms (Meta, Google, Snap, and X) that publish analyzable spending data, according to a new analysis by the Brennan Center, OpenSecrets, and Wesleyan Media Project. Although this is the most complete accounting of online spending to influence the 2024 elections to date, it is an underestimate since no law requires platforms to publish information about political spending. Some platforms publish no data on this, and the voluntary disclosures of others are unstandardized and likely incomplete.

Our new examination of political ad content in the general election period expands on our summer 2024 and postelection analyses of online ad spending, identifying significant differences in the strategies used by spenders. Parties and outside groups were much more likely than candidates to use negative ads, and their ads focused largely on persuading voters. Candidates’ advertising goals, by contrast, tended to be evenly split between persuading voters and fundraising.

Some ELB readers might find this part of the report of particular interest:

There were partisan differences, too: While both sides of the aisle spent on efforts to persuade voters, spending in favor of Democrats was more likely to have fundraising as a goal, and spending in favor of Republicans was more likely to include get-out-the-vote efforts. Additionally, pro-Democratic spenders put a somewhat greater portion of ad money toward contrasting their party’s candidates with their opponents compared with pro-Republican spenders, who spent more on simply promoting their own candidates.