From one of my projects, I'm keeping this way of doing it for future reference, so I won't forget; the project currently uses function pointers instead; but this could come in handy.

Obj-C Selectors.m

double invokeCalculator(id object, SEL method, double price, double extraItemPrice, int index) {
    double returnValue = 0xDEAD;
    
    if([object respondsToSelector:method]) {
        NSLog(@"object responded sucessfully");
        NSInvocation* invocation =
            [NSInvocation invocationWithMethodSignature: [object methodSignatureForSelector: method]];
        [invocation setTarget:   object];                     // index 0
        [invocation setSelector: method];                     // index 1
        [invocation setArgument: &price          atIndex: 2]; // index 2
        [invocation setArgument: &extraItemPrice atIndex: 3]; // and so on
        [invocation setArgument: &index          atIndex: 4];
        [invocation invoke];
        [invocation getReturnValue: &returnValue];
    }
    
    return returnValue; // if it's 0xDEAD I know it's an error
}

// inside the class definition:

// MARK: Private Members
//TODO: finish the actual file-writing part of the method
-(void) writeToFileWithSelector: (SEL) selector price: (double) price andModifier: (double) extraItems {
    double currentValue;
    for (int i = 1; i < 10; i++) {
        currentValue = invokeCalculator(self, selector, price, extraItems, i);
        NSLog(@"%i current value: %f", i, currentValue);
    }
}

-(double)calculatePercentWithPrice: (double)price andModifier: (double) modifier andIndex: (int) count {
    return ((count + 1) * price) - ((count + 1) * price) * (modifier / 100);
}

-(double)calculatePriceWithPrice: (double)price andModifier: (double) modifier andIndex: (int) count {
    return price + modifier * count;
}