SpaceX lists the launch of a Falcon 9 to be $61M. But they can put 13,150 kg into LEO. Rocketlab's is much smaller, only sending 100 kg into LEO. So a Falcon 9 is ~12x more expensive, but delivers more than 125x payload to orbit. Plus you can send stuff into GEO with a Falcon 9, but Rocketlab doesn't have a chance with one their size.

So Rocketlab comes out sounding like a worse deal in the cost/payload ratio, but if Rocketlab can deliver on their launch frequencies then they can capture the market of smaller companies having to buy a secondary spot on a large rocket, and then wait for the whims of the other people to be ready to launch. The smaller company can fork over $5M and have the #1 priority and only payload on the rocket.

SpaceX source: http://www.spacex.com/about/capabilities