I don't think Trump will need more than a few hundred million:
Clinton's unsuccessful campaign ($768 million in spending) outspent Trump's
successful one ($398 million) by nearly 2 to 1. The Democratic National Committee
and left-leaning outside groups also outspent their Republican counterparts by considerable
margins.
But Trump benefited immensely from “earned media” — the free TV time he got by
virtue of being an unconventional candidate who frequently said outlandish and offensive
things. Trump received about $5 billion in free media, according to an estimate by media
analysis firm MediaQuant, compared with only $3.24 billion for Clinton. Those figures aren't
included in OpenSecrets' tally of direct spending.
https://www.washingtonpost.com/news/wonk/wp/2017/04/14/somebody-just-put-a-price-tag-on-the-2016-election-its-a-doozy/?noredirect=on&utm_term=.8a8de4c5e94bThe eventual Democratic candidate will see more than a billion $ spent on
her failure.