Don’t misinterpret the word invaluable. It looks as though it should mean ‘not valuable’, but in fact it means ‘so valuable that its value cannot be assessed’.
The first sentence below therefore expresses a stronger appreciation of Peter’s contribution than the second one:
Peter made an invaluable contribution to our success.
Peter made a valuable contribution to our success.
To express the meaning ‘not valuable’ we say something has no value, or is worthless, valueless or of no value.
Since invaluable may be misunderstood by non-native speakers, it’s probably best to use it only when you’re sure your audience will understand it correctly.
© 2014 Baxter Publishing, Hilversum, The Netherlands