If a word is used by people, then it becomes a word..it exists. Lots of people use the word unravel, as you've pointed out, having seen it in three books. Therefore, it is a word. Language is subject to constant evolution. Look at the number of words now in the language from computer-speak and mobile phone texting. If one person uses a word and another understands what they mean, then it is a word and does exist....
And priceless and worthless aren't, strictly speaking, opposites.