The incumbents are already known. Challengers have to make themselves known, and paid advertising is one of the main ways that is possible.
It's true that (A) the candidate with the most money (B) usually wins, but that doesn't mean A causes B. It's more likely that the most popular candidate attracts both the most money and the most votes, simply by being more popular.
Candidates + parties + PACs + lobbying + think tanks + advocacy organizations spent $10B in 2018 in the US, whereas the US almond industry made $12B in 2018. So I expect political spending to not be able to change ad prices very much compared to commercial ad spending.
https://slatestarcodex.com/2019/09/18/too-much-dark-money-in...
For me, it just made me annoyed at Whitman.
A quick googling gave this¹ doc, that says MW spent $178.5M against $36.7M from JB. Brown won easily.
¹ https://www.mcclatchydc.com/news/politics-government/article...