It might differ based on the company, but the #1 cost by a fair margin for virtually every funded software startup will be the costs associated with employment: wages, benefits, and taxes. That could be 50~80%+ of expenses. (One engineer costs you about $15~20k per month.)

Companies with noteworthy other costs include Zynga/Groupon, which both spend absolutely gobsmacking amounts of money on advertising (and, depending on how you count revenue/expenses, their revenue share deals with Facebook and merchants respectively).

Server/hosting expenses tend to be heavily sensitive to what you're actually doing: Zynga and Dropbox pay a lot relative to their revenues, but at a 37Signals-esque company they're chump change.

Legal fees are typically rounding error. Ball up all administrative overhead and that's in handful of percent region.

Your physical workspace can be a fair chunk of change, depending on where you are, how you came by your space, and to what degree you optimize for having a prestigious address or office space.