Tens of millions of dollars for hardware, and millions more for manpower is expensive for most groups in the world.
You couldn't even start on something like this with less than $2,000,000.
Several orders of magnitude more computing power than the rest of the world combined to manipulate the chain? Hmmm. One thing is clear, you only need 50% or less of the total mining capacity (supposedly currently worth 50 million dollar or less) [...]
You guys forget one thing: Currently bitcoin is rewarding 50 extra coins per block (worth roughly 650$), that's what pays for the quite large current total hashrate of about 11,000 Ghash per second.
These 50 blocks are rewarded only temporarily, bitcoin is designed to run on transaction fees only, currently roughly 0.13$ per block.
Please redo your calculations based upon a hashrate that is about 500 times lower.