Have time to spare?
Try taking an overnight bus from JB to Hatyai, should be around rm40-60 or a little more.
From Hatyai, you can either take the train or bus to various parts of thailand, if game, take the train to bkk, or go by one of the public buses. Its relatively cheap and plenty to see. Can't remember the cost, but it was pretty reasonable, under sg$200 because I stopped at various places.
If you are sick of the long travel, then you can take the thailand budget airlines, such as 12go up to changmai and beyond.