Why add holidays and sick leave to an individual employee's costs? Most white collar employees would have an annual salary this is factored into already, unless you're having to hire temporary replacements for them perhaps (which would be more than a 15% cost, I suspect).
You are right, I usually do this to get to a worked daily or worked hourly cost (I work in consultancy) so I take holidays & sick into account to understand cost per worked hour / cost per worked day.
In reality it depends on the type of work you are doing.
Actually there is a more important comparison than salary: what benefit accrues to the company for 1 employee month.
When a company is undergoing fast growth, the return from one month of work can be startlingly huge. Every tool that enables better use of time is a multiplier on the income generated by that employee.
Also note, “sir” seems snarky to me at best and insulting at worst. Are you even answering a guy?
15k could be cost to the company, which also looks at employment taxes, office space, health insurance, benefits, hr/support overhead, interviewing, training and managing. A new developer earning 8k/mo easily costs the company 15k/mo.