Trump becomes the “presumptive nominee” the day he announces his candidacy. So no campaign budget needed for primaries.
OTOH you could definitely see a prolonged contest on the Dem side with a very weak Biden or Kamala, and the usual suspects from the last time around.
He can win if computer voting machines are banned and vote harvesting and finding votes after the Democrat is found to be losing.
Trump won 10 million more votes in 2020 than 2016 and he can do it again.
I wouldn’t mind him having a primary to stay sharp and get back into jousting