@Michelle87 Just write them back and say something like "Our base weekday rate is $XX and our base weekend rate is $YY. Five nights at $XX and two nights at $YY would be $ZZZ without the weekly (monthly) discount. The quote you're seeing takes the discount into account." If they don't "get it" after that, I'd simply decline them. They're not the kind of guests you'd want to mess with anyway.
One last thing... don't attempt to show math calculations in your message. AirBnB will interpret that as either phone numbers or URLs, and they'll redact them. Plain English as I've shown above will not be redacted.