Again you must define the parameters your thinking about when you ask that kind of question because you will get a whole host of answers, which while they are accurate for that individual’s situation, they will not give you any insight on what to expect when you attempt to start up a studio of your own.
From my personal experience working as a contractor in software development for other’s which include those that have a small office (~30’ x ~30’) the monthly costs was about ~$7,000 a month for them, that included my monthly salary, 2 other guy’s monthly salaries (rates were between $15.00 to $20.00 an hour), the office rent, utilities and some monthly license fees. Their revenue was between $5,000 to $13,000 in sales, retainer fees and consulting fees a month, that was between 2004-2007.
Now just based on that, if we consider a modest studio size, with 3 coders ($6,000 x 3 = $18,000 monthly salary), 4 artists ($6,000 x 4 = $24,000 monthly salary), and a system administrator ($6,000 a month), studio rent ($10,000 a month), plus utilities (~$1000 a month), with competitive salaries implied this would be between $55,000 to $65,000 a month to operate or $660,000 to $780,000 a year to run.
So I will say it again, do not worry about trying to “look” like a true game development studio until you actually have a product or two that sells! When you have a steady stream of revenue coming in through sales, license fees, contract work or profit shares from joint projects, THAN you can think about growing into a studio.
For me I operate out of my home, I budget between $0 dollars to $1,000 dollars for assets, per game based on what I need and I am the sole developer coding the game play. I plan on releasing a game later this year or early next year. Whether it actual generates modest or strong revenue is not as important as my being able to afford my overhead costs and not jeopardize my current standard of living OR risking my day job.
Now if the games I release actually sell and I make enough to where I MUST buy the pro license of Unity (I would love to have that problem) I will still keep my day job and will simply create more games, with a slightly higher budget and see where it takes me. If my games continue sell to a point where my yearly revenue stream from game sales is equal to or greater than my day job, THAN I will consider quitting and going full time game developer.