A standard argument in economics is that retailers set their prices just under whole numbers, say $9.99, to force their employees to open the till for change, reducing the chances of them pocketing the bill or because they think, mistakenly, that consumers will somehow read more into the one cent price reduction than a one cent price reduction, all because the first number changed. Economists assume that consumers aren't actually fooled into buying a lot more at $6.00 than at $5.99, because honestly, who's that stupid?
May be the French. According to this this story from the BBC
... according to a French study the phenomenon still swings a considerable number of shoppers. Researchers found that lowering the price of a pizza from 8.00 euros to 7.99 euros boosted sales by 15%.The BBC report says that Robert Schindler, professor of marketing at Rutgers Business School in the US, argues 0.99 idea was introduced for sale items, to emphasise the discount.
"I studied adverts in the New York Times from 1850 - where there were no 99 endings - to the 1870s and 1880s where they started to appear. Although department stores were doing it - which would fit with the cash register hypothesis - they were advertising discounts. But for the regular price they would use a round number," he says.
He thinks the retail practice developed from there, to communicate discount or the impression that things are on sale - even when they are not.