So he's currently using about 10TB of 60TB of usable space. If he uses Amazon S3's standard storage, he would be paying about $230/mo. If he uses infrequent storage that is $125/mo. That goes up as his usage goes, so when he's using 30TB that will be $690/mo and $375/mo respectively. He also has the benefit of high speed ethernet with the home NAS, unless he has fiber 1Gig internet, in which case speed is probably a wash. I'm not sure if there are other significantly cheaper cloud storage solutions at that scale.
So I'd say he hasn't done too badly for himself, though he probably could have saved some on the hardware by getting cheaper parts.