What you need depends on where the conference is held, airfare costs to get there, hotel rates, the price for meals, and registration fees. For local conferences, you may only need to pay a registration fee. A conference held in New York, Boston, or Washington, D.C., on the other hand, will require much more from your wallet. If you don't have enough to finance the entire trip, there are places to look for assistance. The most obvious place to look for help is in your department.
Most departments have some funds available to pay for travel to professional conferences.
Don't hesitate to ask your adviser for help. You're much more likely to get support if you're going to participate at the conference. Matt White, who received a Ph.D. in economics, attended four industry conferences related to his research on market structures and public utilities while at Berkeley. His appointment as a graduate student researcher provided research funds to pay for most of them. Tali Carmon, who earned a Ph.D. in industrial engineering and operations research, attended five conferences as a graduate student with departmental support and says whatever she had to pay on her own was worth it.