[hansvm]

That's less true as salaries increase. Some of the increased costs like matching social security contributions or 401k matching are proportional to salary (to the tune of ~8-25% in aggregate depending on a ton of factors), and many of the other large benefits like health care are fixed and consequently take a much lower percentage of a large salary (10% would be a large overestimate for total employer health care costs for the salaries listed).

"Double" is a much better (but still probably a bit high unless you're also factoring in HR and other similar overhead) estimate for low-paid white-collar workers. Lots of fields in vast swathes of the country start out around $30k/yr but still have full benefits. A $3k-$15k/yr healthcare benefit on top of the other variable costs can push you close to double.