I've said it before but it bears repeating.
In no way is asteroid mining cost effective at the moment. However, that's not actually very relevant.
Several companies are working toward bringing down launch costs, SpaceX being the farthest along. If such efforts are successful they could result in lowering launch costs by around a factor of 10 to 100. That fundamentally changes what it is possible to do in space. Which obviously has a very direct impact on the feasibility of asteroid mining. More so, it will probably spur development in Earth orbit as well, including things like inhabited orbital stations, hotels, and eventually cities. That will create a hungry market for materials in orbit, and there the advantage of mining from an asteroid vs. launching from Earth (even given the reductions in cost mentioned above) makes asteroid mining all the more competitive.
Additionally, advances in robotics and automation could potentially make asteroid mining very cost effective. Consider that when mining an asteroid there is no environmental concern and the most effective mining methods can be made use of with brutal efficiency. And, of course, the development of advanced robotics to facilitate asteroid mining would of course be valuable for other uses and thus be a profitable spin-off invention.
There are other points of interest but they are relatively less important. The core point is that if these developments happen then asteroid mining could quite easily become profitable. Of course, there is no guarantee that any of this will happen. So why not wait until the opportunity is ripe and then start? You certainly could, but you'd find that you're just enough steps behind Planetary Resources to be at a significant disadvantage. They'll already have experience with operating spacecraft. They'll already have developed the core technologies necessary for mining asteroids. They'll already have identified the most valuable asteroid targets. They'll already have run pilot operations and overcome several key roadblocks. And on the day that it becomes profitable to sell asteroid mined resources they'll be there with the goods. And if it turns out not to be feasible then overall they'll still have made a better use of their funding than, say, buying instagram, and they likely will not have spent nearly as much money either.