Freelancing in Brussels

In round figures, you keep half (or a bit more) and the rest goes to social security payments and taxes.

It is so dependent on personal circumstances and on your turnover that it's nigh on impossible to answer. Start by getting advice from an accountant or somewhere like here:
http://www.parcourscrea.eu/
Or try getting advice through ACTIRIS

Best is advice & assistance from any Guichet d'Entreprises (for example from Partena etc.) or Ondernemingsloket (for the dutch-speaking areas).
you will need an acknowledgement of your professional education from the Belgian state, a VAT number etc.
Best is always with the 1st of new quarter (1.4. / 1.7. / 1.9.) to avoid higher social security costs referring back to a running quarter. Count with a minimum of ca. 500 EUR/quarter for social security - even you will not have a single cent of income, as these costs are obligatory.
Keep all bills related to your getting independant, set-up an excel-sheet for basic accounting, get an accountant - and get clients.
Good luck!

It is exceptionally expensive to be self-employed in Belgium, with miniscule medical insurance & pension.
but since new VAT regulations, you can now set up a simple small company in your own [european] country with a VAT number and issues bills to clients here & other european countries. The costs lower & insurance & pension benefits higher make this the way to go about it.
Good luck

Or you go for a payroll mgt co. I've used Tresag (www.tresag.ch or .com). You get to keep about 65% with them