I have thought about the same thing when playing with deep learning at home, until I put electricity bills into the equation. A beast like what the author has built, with full power, will cost $0.1/hour in electricity bill alone at where I live ... which almost equal to the cost of a p2.xlarge spot instance on Amazon.
So if someone else is paying for the electricity, build your own gig, otherwise, Amazon is pretty hard to beat.
So if someone else is paying for the electricity, build your own gig, otherwise, Amazon is pretty hard to beat.