Don't know about a cheap flightplanningsystem. We use an airlinesystem but you can send your flightplans direct to the briefing in Brussels via <a href="http://www.belgocontrol.be" target="_blank">www.belgocontrol.be</a>
You can also test your IFR flightplans on the CFMU website:
<a href="http://www.cfmu.eurocontrol.be/chmi_public/ciahome.jsp?serv1=ifpuvs" target="_blank">http://www.cfmu.eurocontrol.be/chmi_public/ciahome.jsp?serv1=ifpuvs</a>
Are you operating out of Kortrijk?
Rgds
Deing