From the Cook Political Report (paywalled):

In the 20 most expensive House races, Democrats outspent Republicans in all but two races, per AdImpact. That’s largely due to Democratic candidates’ fundraising advantage over Republicans. Across the 20 most expensive races, the median Democratic spent $5.8 million on ads, while the median Republican spent $2.5 million.

The Congressional Leadership Fund attempted to make up for Democrats’ advantage, providing “on average over 50% of the GOP’s broadcast TV presence” in the 20 most expensive contests. But that wasn’t enough to close the gap in most races.