The {!
pair is a delimiterstarts 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) }">