I am also looking into a scrypt mining rig and I've been doing the math also around the power-unit and energy costs.
At first I thought I'd buy (3x) a 280x, since they are a bit cheaper and have decent hashing and not-too-high-power consumption. But to be on safe side, you would need at least a 1200W PSU. If you use other cards that are more power-hungry, that might not be enough in a tripple-GPU setup.
So I am considering the more expensive (older) 7950's, for which I can get away with a 750 or maybe 850W psu.
In the end, the price is the same BUT: there is a 30% higher power consumption for the 280Xs. At 3 times of them over one year that is a alot of money.
Mind you, this is a rough calculation, but still.
Not that I am counting on getting my money back really, it's just for fun, but if one can save a bit for the same performance....
As probably pointed out, the example build-rig is probably one of the cheapest you will find. Of course cheaper with only one GPU and you won't have to worry about an expensive power-unit. But still, a 20-30 dollar (euro) more expensive older 7950 if you can find one will save you a bit in the end via electricity.
Here is an example for a single GPU and a tripple GPU:
http://www.forbes.com/sites/jasonevangelho/2013/12/09/litecoin-101-how-to-build-your-own-mining-rigs/2/This is a few months older and can be "converted" to a single or dual GPU rig, I guess (then you can get a cheaper motherboard)
http://www.cryptobadger.com/build-your-own-litecoin-mining-rig/