IMHO, a reasonable estimation for fully loaded cost per employee (excluding facilities expendetures) is approximately 1.4 * ES, where "ES" is the employee salary.

The "three DevOps engineers and save $10k" estimation was based on working backward from the 92% of available jobs in Boston being less than half of the stated $20k per month cost. Assuming a Gaussian distribution where 0.5 * $20k per month represents the high end of two standard deviations (since Boston ranks quite highly in S/W salary nationally), most DevOps engineers will be paid roughly half of that as well.

This yielded an estimation of $6.5k per month per DevOps employee or $19.5k per month for three.

Since all of this was off-the-cuff, I figured it best to throw in a bit of "fudge factor" and present a $10k per month savings.

As always, YMMV and I could be completely wrong about all of this :-).