Thank you for the detailed response Danny. 

I guess a rough cost estimate could be obtained by determining the value of bitcoins mined (around half a billion $ per year?).   The mining operations cost less than this to run per year or it wouldn't be profitable.  Yes perhaps the payback period for the equipment is several years but that is not what I recall hearing.  If the attacking entity decided to sell the coins they mined they might offset most of that cost.  But assuming it is an inefficient govt agency, and they don't even sell the coins mined, it looks like 51% of the network computing power could still be obtained for under $1 billion.  That is less than the average fighter jet costs - to save US dollar hegemony worth trillions.

The question of "could it be blamed on another entity" is very important imo, since public backlash currently makes an overt attack politically unpalatable or perhaps impossible.   However the govt has proven very willing and able historically to engage in secret operations that would otherwise be rejected by the public. 

My main question now is:  If a government agency developed a "mining" operation equal to 51% of the network, could it be brought online to appear as just more private miners - or would there be clues that the operation was concentrated in a few locations with massive computing power in each location?