Benefits, office space, equipment, T&E, employment taxes, etc. This is an old article but it suggests about 2.7x. [1] I agree that it might be less for a startup assuming a frugal startup but it's probably at least 2x salary on average.

[1] http://web.mit.edu/e-club/hadzima/how-much-does-an-employee-... (This is old but nothing really substantial has changed except.)

I'm not sure a ratio-to-salary is the best tool here. I mean, why would a developer on $150,000 have twice the healthcare cost, or office space cost, or equipment cost of one on $75,000?

It’s just a rule of thumb and some costs do roughly scale. I’m sure there are more sophisticated calculators and spreadsheets for this sort of thing.

I'm assuming you don't run a business. 2-3x sounds in the right ballpark to me.