Edwin, it really is a case-by-case matter. Sometimes CTO will be worth the same as used - usually when genuinely postally used are rare to barely existent (as with some of the Sand Dune countries); other times, as with the Australian issues I mentioned, CTO are probably worth less. You'll need to study the catalogue you're using carefully for clues.
Sometimes, it takes a specialist to pick the difference. The two stamps below are from the 1931 pictorial set from Charkhari State in India. The first stamp is CTO, and basically completely worthless. The second stamp was actually postally used; if I was selling it, and I'm
not, I'd ask maybe $50 for it:

You won't find the difference between CTO and postally used in any catalogue. You just have to
know.