A difficult question to answer, not knowing what you mean with 'starting his (or her) career', you may see answers such as; nothing, €35 per block hour, €7000 per month.
You see, it depends on so many factors: If you are in Line Training or not, your hours on type, hours you fly per month, contractor/RYR employee, if contractor; your agent & your personal tax arrangements, if employee; your base & hours.
If I was going to give you an estimated net income (yearly net income divided by 12) for a three year tax compliant FO; 3000 euro/month.
To understand all the variables though you kind of have to be in the game..