Responsive (and real-time) websites are fundamentally more complex and more expensive to build. There’s no getting around that. If you're going to make the leap, it’s a business decision like any other, and you need to decide if it’s worth the significant initial investment and ongoing maintenance cost.
The apples to apples comparison would be a responsive site versus two separate sites for mobile vs. non-mobile. In that case, I'm not sure its clear a responsive design is fundamentally more complex and expensive to build. In the latter case, every time there is a new feature you have to essentially implement it twice.
