It is the toughest question to know how much money you need before you are ready to apply for graduate school. This part will only discuss application for graduate schools in US. The number should be taken as an average cost; real cost will vary from case to case.
You need to take into account these expenses at the very minimum:
GRE: $160 (free to send to 4 institutions, $15 for additional score recipient)
TOEFL: $140 (free to send to 4 institutions, $17 for additional score recipient)
Application fee: $30-$60/schools (apply at least to 3-5 schools)
At the very minimum you need ~$350 for one school; realisticly you will need ~$500 for your applications. Note that this is the minimum cost.