The salary of EU officials, civil servants, temporary agents and contract agents is set out in the Staff Regulations and multiple factors are considered. These include the official's seniority, work experience, the exam (s)he had passed and on the basis of which (s)he was recruited, child allowance, the place of recruitment (expatriate allowance) and other factors.
The average net monthly salary of an Assistant at AST3 level is around 3200 euros, while for an entry level AD5 Administrator would earn around 4500 euros.
See also:
CAST, Contract Agents, Temporary Agents, EU Officials, Recruitment, Open Competition, AST1-AST3, AD5-AD7-AD9
Related link:
New To EPSO Competitions and Recruitment Exams?, EU Agency Selection and Recruitment Database