I don't know how much we've spent on boost phase program of missile defense. As for the article itself and the study it cites, a lot depends on the architectural assumptions of the study. Without having read the study, I can't comment on it directly. However, this paragraph from the article deserves some comment:

Second, the study notes that liquid-fuel missiles burn out in four minutes, solid-fuel missiles in three. Subtract from that the half-minute or so it will take to confirm a missile launch, and a boost-phase interceptor doesn’t have much time to reach its target. The interceptor will have to be very fast and very agile. Both characteristics will require it to carry much fuel, which requires it to be quite large — larger than an ICBM, the study calculates, and able to accelerate four times as quickly. “Such interceptors have never been built,” the study notes, “and would push the state of the art.” They would also require fairly enormous satellites to carry them.

I agree with the prior paragraph that an orbital constellation of interceptors would require a lot of satellites. Calculating how many satellites depends on a lot of variables as defined by the architecture. For example, if the study assumed that the interceptors must be able to chase down the missiles from behind, then they're right that the interceptors would have to be huge and include a lot of propellant.

However, I believe it is possible to design the orbital constellation where an energy-wasting stern chase wouldn't be required. This might be possible by designing the constellation so that the most likely missile flight paths could be intercepted head-on or nearly so. Doing this would probably require putting interceptors into several orbital planes, most with high inclination. I visualize something like the Iridium constellation, only with more orbital planes and satellites. In the event of a launch, the system would calculate which interceptor has the highest probability of a kill and it would be fired.

"If a senior scientist says something is impossible, he is almost always wrong. If he says something is possible, he is almost always right."
- Arthur C. Clarke