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The {! pair starts the expression (and } terminates the expression) rather than the ! character negating the expression.

So use e.g.:

<aura:if isTrue="{! notequals(ch.sumchans__StandalonePrice__c, 0) }">

or:

<aura:if isTrue="{! !equals(ch.sumchans__StandalonePrice__c, 0) }">