Making a website that works on desktop, tablet, and mobile all at the same time is difficult and expensive. Responsive Design's value proposition is that it is somewhat less complex and expensive than having a bifurcated experience on an m. subdomain. It also avoids some issues attendent with bifurcated experience[1], and maybe you can even have a decent tablet-specific design.
The head of webdev for my company says that a responsive design is usually about 20-30% more expensive than a traditional mobile-ignorant design. This is significantly higher than 0%. It is also significantly lower than the ~100% increase it would take to build a separate dedicated m. experience. The resulting mobile experience is significantly more full-featured than the shitty bifurcated experience[1] you could otherwise get with 30% of the main site's budget.
Responsive Design is not a silver bullet, and should not be sold as such. If it is worth your time to make a mobile version of your site, it is an extremely attractive option for doing so, and generally superior to the alternatives. Doesn't mean it's free or perfect, just means it's better than the other ways of doing it.
A bit OT, but I don't really get the need for a tablet specific design apart from the desktop site in general (I'd see why gestures for e.g. would be tablet specific, but almost no site with iPad specific versions use them). What would be the hurdles to have the same version for tablets and desktops, and a more reduced version for mobiles? It would force a very discoverable page design, no plugins required, and a lot of side benefits from the design POV in my opinion.
We used gestures and a whole slew of iPad-specific media queries and rendering logic on bostonglobe.com. While I agree that you can usually write a lot of common desktop/tablet code in a responsive design, adding tablet-specific (or iPad-specific) features in a responsive design is a big selling point to your typical executive-type.
(Dislaimer: I used to be an engineer at the Boston Globe)
In my experience it's 30-40% more expensive, but I try to make the responsiveness fluid from min to max, instead of targeting certain breakpoints so I might be spending a bit more time than others.
Unless you're really going crazy with your dedicated mobile design, it's not going to cost 100% more than a desktop design. I suspect that it many cases, it's probably the same 20-30%. In my mind, the main benefit of being fully responsive is that (in theory) it's easier to maintain a single code base and (in theory) it's more future proof.
The head of webdev for my company says that a responsive design is usually about 20-30% more expensive than a traditional mobile-ignorant design. This is significantly higher than 0%. It is also significantly lower than the ~100% increase it would take to build a separate dedicated m. experience. The resulting mobile experience is significantly more full-featured than the shitty bifurcated experience[1] you could otherwise get with 30% of the main site's budget.
Responsive Design is not a silver bullet, and should not be sold as such. If it is worth your time to make a mobile version of your site, it is an extremely attractive option for doing so, and generally superior to the alternatives. Doesn't mean it's free or perfect, just means it's better than the other ways of doing it.
[1] http://xkcd.com/869/