Invaluable and valuable are nearly synonyms, despite the potentially misleading negative prefix in- on invaluable. Valuable means of considerable value, and invaluable means incapable of being valued. If you need an opposite of valuable, there are many options such as inexpensive, cheap, and worthless.
The difference between invaluable and valuable can be subtle, but think of it this way: valuable usually applies to things that have monetary value, while invaluable usually applies to things that can’t be valued in monetary terms. For example, a hoard of gold is valuable, while a good friend is invaluable (i.e., can’t be valued in monetary terms—hence the negative prefix in-).
Both words are about five centuries old, deriving from value around the early 16th century (value came to English shortly before from Anglo-Norman and Middle French roots). The Oxford English Dictionary records examples of invaluable used to mean having little value from the 17th and 19th century, but this sense has never been widely used. In historical Google Books searches covering the 18th and 19th century, every instance of invaluable that we find is in the sense incapable of being valued.
Google was getting used to wearing the crown as the world’s most valuable brand. [San Francisco Chronicle]
His answers, and his profiles of “pioneers” who have made such transitions, make for invaluable and inspiring reading. [Wall Street Journal]
A case could also be made that state-controlled Chinese oil company PetroChina was briefly more valuable than either company. [CBC]
Having designed gardens for nearly 40 years, I have accumulated an invaluable list of artisans and suppliers. [Telegraph]
The finest were made from precious materials and were among the most valuable gifts bestowed by princes and monarchs. [Mirror]
Having music feature in a primetime TV programme can also mean invaluable exposure for local artists, both here and overseas. [Stuff.co.nz]