SN Brussels Airlines have a flight from Bristol at 1100. (Arrives Brussels at 1310). For the return flight you can choose between a departure at 1540 (arrives Bristol at 1550) or at 2140 (arrives Bristol at 2150). The inclusive fare is �117.70. The 1404 train from the airport arrives at Brussels Midi at 1425, where you can pick up the 1520 TGV to Lille Europe, arriving at 1556.
To get back for the later of the two return flights (which would allow you time to explore the stunningly beautiful city of Lille), you could take the TGV at 1758 from Lille Europe (arriving Brussels Midi at 1833). Then connect with the 1848 train to the airport, arriving at 1908.
TGV services have to be pre-booked. The return fare is �37.
That makes the cost of travelling on the Bristol - Brussels flight come out roughly as follows (assuming you drive to the airport and ignoring parking costs):
Petrol about �10 + Flights about �118 + trains to/from Brussels airport to Brussels Midi, about �10? + TGV �37 =
about �175 by air to Brussels
It seems that supporting MUFC will be fairly expensive, however you do it. That is why you're going, isn't it? ;-)
Links:
National Rail (for pre-booking rail travel to London):
http://www.nationalrail.co.uk/
Eurostar:
http://www.eurostar.com/
easyJet:
http://www.easyjet.com/en/book/index.asp
SN Brussels:
http://www.flysn.com/com/home/
SNCF (for booking TGV services):
http://www.sncf.co.uk/
Chris