It's in the books, under "battle-forged armies" Then the subsection titled: 'unit champions'
In Xenos 2, this is on page 126. They have the same cost as any other model in the unit.
If there are exceptions, they'll be called out. Imperium 1 specifically mentions Wolf Guard Park Leaders in Terminator Armour do not follow this rule.
I assume the Chaos book also mentions the sorcerers for Rubrics and Scarabs.
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