How to travel by train from paris to brussels cheap?

I'm on a tight budget, so don't think I can take the TGV trains. I read on the internet that there are local slow trains from paris to lille, and then onwards by metro to belgium border, and then train to brussels. Can anyone give me some details on this? What will the rates be like? I don't mind stopping at lille for a few hours to have a look around. Also any alternate routes, or bus routes are also welcome