This analysis assumes 100% utilization: keeping the machine busy 24/7/265. It also ignores AWS price drops and the value of being able to switch to new AWS GPU instance types as they become available.

TL;DR: Proposed machine costs $3k plus ~$100-200?/month for electricity, comparable AWS is currently $3/hr.

My conclusion: If you're going to do more than 1-2,000hrs of GPU computing in the next few years, start thinking about building a machine.