IMO the short answer is, yes pardons can seemingly be bought in the Trumpian era of American politics. Whether $1B is enough, I don't necessarily know, but I don't see why the likelihood is something to dismiss out of hand.
1B gets you in the door. You still need to know the right people to pay off. You can't just google it. This is why you see all the billionaires at their billionaire parties, good old networking.
https://en.wikipedia.org/wiki/Bill_Clinton_pardon_controvers... https://en.wikipedia.org/wiki/List_of_people_pardoned_by_Bil...
You should read Swiss investigative journalist, Daniel Amman’s King of Oil.
The furor over these pardons including the Marc Rich pardon was 100% political.
Clinton pardoned relatively the same amount of people as previous presidents but with a heavy focus on “leftists.”
Also, Marc Rich had left the US before he started working across trade embargoes and singlehandedly built the OTC oil market. He also worked actively with the US on covert oil sales while being prosecuted by the DOJ.
Trump tends to pardon the people who will get him into the news cycle, like sadistic war criminals, corrupt or openly racist officials like Blagojevich and Arpaio, or whoever Kim Kardashian told him to.