How much would it cost to do a 51% attack
There is a handy chart at https://en.bitcoin.it/wiki/Mining_hardware_comparison.

If we assume that it is complete, and that the figures listed there are accurate (I have not verified them), and that the attacker must buy the hardware, then the most economical device is the Antminer S5+, which produces 7.7 TH/s (7.7e12 H/s) and costs US$2300.  It draws 3.4 kW.

The current network difficulty is about 5.6e10.  This means that an average of 5.6e10 x 2^32 = 2.4e20 hashes are needed to mine one block.  Assuming that one block is mined each 10 minutes (600 seconds) on average, the network as a whole is producing 2.4e20 / 600 = 4.0e17 H/s.

So a 51% attacker needs to produce about 4.0e17 H/s himself, to match or slightly exceed the hashing power of the existing network.  This means he needs 4.0e17 / 7.7e12 = 52000 Antminer S5+ devices, which would cost 52000 x $2300 = $120 million.

(This is unrealistic in a couple of ways - buying that many devices would not only drive up the price but probably exceed the production capacity.  On the other hand, such a large order might qualify for a quantity discount or wholesale pricing.  This attacker might also find it more efficient to license the design and do the manufacturing themselves.)

Running these machines would consume 52000 x 3.4 kW = 177 MW of power.  If the attacker is located in the US, this table shows that the cheapest place to get electricity at an industrial rate is Washington State, where it costs an average of 4.5 US cents per kWh.  At this rate, the attack would cost US$8000 per hour to supply with electricity.

If they are all to be located in one place, there will be additional costs to build / rent and operate a facility to house them all, with appropriate electrical infrastructure.  There will also need to be significant network and server infrastructure to connect them all to the Bitcoin network.  Cooling costs will also be substantial - Google says that their data centers have electrical overhead of about 12% (i.e., on top of the electricity used by the equipment itself, an additional 12% is used for cooling, power supply losses, etc).  They say that the industry average is more like 70%.  You would also have to hire an IT staff to maintain everything.

This overhead might be reduced if the devices were to be distributed (e.g. each of 52000 people keeps one in their house) but then you start to incur administrative costs to keep all those people organized.  (Just picture the communications department to tell them what to do, accounting to make sure they all get their cut of the proceeds from this evil scheme, human resources to recruit new participants, etc.)