Cost of matching mining power?

I did a quick calculation of the approximate cost it would take to buy enough off the shelf hardware to match the current hash speed of the network (5451.42 Gh/s according to bitcoinwatch). Based on the cost of my mining rigs, I would estimate it to cost around $9 million. Accounting for a healthy margin to get you well over 50% and the infrastructure it would take to support that hardware, I'd say for an initial outlay of $30 million you could have enough hardware to outpace the rest of the miners on the network. Now, that is like 15,000 high end AMD cards...I'm not sure whether they would be able to meet a demand like that...but, for a motivated and well funded entity that had an interest in destabilizing bitcoin, this doesn't seem like a huge number (of course, by the time such a system could be built out, they might very well have to double down again on that outlay to keep up). Is my math off? Should this be a cause for concern?