Yeah, the ThreadRippers are amazing. I was truly astonished when I noticed that one hour of V-Ray on 3970X outperformed $1000 in ChaosCloud rendering.
At a price of $2500 once for the TR versus $24,000 per day for cloud, it's a no-brainer to buy them. Sadly, availability in my area is pretty spotty and even Amazon currently predicts 2+ weeks of delivery time.
For EC2, one would need to rent licenses for the V-Ray renderer plus V-Ray tends to be heavily CPU-cache- and memory-bound, so exactly the parts that get slow if you enable the Intel security fixes. Due to c5.9xlarge being Xeons, they are like 80% slower than a comparable AMD CPU.
BTW, calculator.s3.amazonaws.com quotes me $1119 per month for a single c5.9xlarge, so I'm not sure how you would reach $8/day when I see $37/day.
At a price of $2500 once for the TR versus $24,000 per day for cloud, it's a no-brainer to buy them. Sadly, availability in my area is pretty spotty and even Amazon currently predicts 2+ weeks of delivery time.