It comes down to incrementality. Sure Trump has way more money than most challengers, but most people have also already made up their minds about him. His cost to acquire incremental voters is probably much higher than a lesser-known challenger.
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...
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...