At first glance, the terms valuable and invaluable seem to be opposites. However, they are very close to being synonyms even though the latter term has the negative prefix in –. Valuable denotes the idea of something that has considerable worth, whereas invaluable gives the idea of something that is so valuable that its value cannot be quantified. Terms that are opposite of valuable or invaluable would be worthless, inexpensive, or cheap.
The term valuable can be used to describe an item that can be ascribed a tangible value. For example, a diamond that is worth $10 million can be said to be valuable. However, items that cannot to be assessed with a monetary value such as friends, family, or good health are considered invaluable. The prefix in – is put on the word invaluable not to signify its lack of value, but instead to denote that the object being described cannot be valued using monetary terms.
It should be noted that there was a time during the 17th century when the term invaluable was used to denote something with little value. However, that definition was rarely used, and starting in the 18th century onward, invaluable has only been used as a way to describe something that has a value that cannot be measured in monetary terms.
All that being said, the average English speaker is going to understand the terms invaluable and valuable to mean the same thing. It is not uncommon to hear people refer to their loved ones, something that cannot be defined by a monetary value, as being very valuable to them.