Invaluable Consideration

Contrary to initial appearances, the word invaluable does not mean worthless or without value. Although the in- prefix adds "not" to the word valuable, you have to take into consideration the structure of the word.

Valuable essentially means "of worth", so adding an in- prefix would create a word that means "not of worth". That seems pretty straightforward, but it isn't what is happening here.

Invaluable actually consists of three parts: the prefix in-, the root value, and the suffix -able. When you define value as "place a price on", the meaning comes clear. An invaluable service is not worthless but one on which a value cannot be placed, one which we are not able to value. Thus, priceless and of inestimable value work as synonyms. Why you would choose to use the latter when the word invaluable means the same thing and requires fifty percent fewer characters, I couldn't say.