I object to the idea that a "few million bucks" would place a person in control of mining capacity large enough to be 51% of the network.
Between video cards, computer hardware, cooling, OFFICE SPACE, labor, advertising to get that much labor, electricity, etc. it would have to be quite a chunk of change. A few million probably wouldn't do it.
In another thread I estimated that it would be ~$2M per TH with COTS when you consider labor, electricity (including mains upgrades), warehouse space, racking, cooling, and administration.
However I personally believe the network is too large to be sustainable at this point. In the medium term I expect hashing power of the network to continue to decline as there is insufficient real transaction volume to warrant the current network size.