the number four connects to "death" (secondary word meaning)
for this reason, no 4th floor sometimes, no four thousand yen.
"nine" means "suffering" secondary.
"999" worse than "666" (number of the beast).
in 1666, there was a fire in london, and it might scare british people.
here a list:
4,9,13 40,44,99 144 (12*12) 444,999 4000 2999,3999 etc.
now i have given illustration why prices are 39,98 most likely.
it lets people believe, that the price would be "thirty". conspiration theory of course.
good numbers: 3,5,8,10,12,1000,5000