@Gilles already has a very nice answer and mine is on the same line, although the route differs. I have travelled a lot between brussels and Paris avoiding the Thalys by train.
The quickest route involves some high speed connection, from Lille Flanders to Paris. Either on Tuesday or Thursday the SNCF publishes last-minute offers. It has been a while I have been monitoring their offers, that is why I can't be precise on it being tuesday or thursday.
The connection between Lille and Paris is quite often mentioned as a last minute.
For both of my suggestion you need to get into Lille from Brussels.
1. As Gilles already mentioned you can use ticket, but there is a cheaper solution in the Railpass (>26 years old) or the Gopass (<26 old). These two passes entitle you to 10 unlimited trips in Belgium. As such only relevant if you plan to do more trips in Belgium.
So depending on your age, you can get to the French border for either 5 or 7 Euro. From the Border you need another ticket.
If you secured a "derniere minute" offer from the SNCF you need to buy a ticket from Mouscron to Lille (6,20 Euro). While writing this answer I could get from Lille to Paris by TGV for 25 Euro for tomorrow, so it is definitely worth looking at.
When I didn't find a good offer for the tgv, I went through Maubeuge. There is a train going to Maubeuge from Lille every hour and there are frequent direct intercity's from Maubeuge to Paris (Gare du Nord), which costs 45,80 Euro.
So would I need to travel tomorrow I would pay for
1. 7,10+6,20 + 25 = 38,30
2. 7,10+6,20+ 45,80 = 59,10
As you can see my route through Maubeuge is slightly more expensive then @Gilles route through Amiens if you use the railpasses and border crossing ticket in stead of buying one ticket form Lille to Brussels.