Price and worth aren't always exact synonyms to begin with. "Worth" has several definitions. In this context, consider the definition of "worth" as the abstract idea of value or merit. Price, meanwhile, is the concrete quantification of value in terms of money. So worthless = "has no value" While priceless = "value cannot be quantified" If something has no value, its value can easily be quantified as 0. So worthless and priceless are clearly not synonyms. But they're not exactly antonyms either -- if something is priceless, that doesn't necessarily mean that it has infinite (or extremely high) value. It means that it's not something to be offered for sale. The idea of buying and selling things with money is all based around the assumption that goods can serve as substitutes for one another, which doesn't work when some item has truly unique properties. More on reddit.com

That's because worthless and worth full are antonyms More on reddit.com