As you probably know, medications available in generics are generally quite a bit cheaper than those that aren't available as generics. Here's a brief rundown of some popular Crohn's meds and their relative costs:
Remicade, Humira, etc: very expensive ($ thousands per month)
Entocort: very expensive ($ hundreds per month)
Pentasa, Asacol, etc: very expensive ($ hundreds per month)
Azathioprine, 6-mp: Cheap, if you get the generic ($ tens per month)
Prednisone: Cheap ($ tens per month)
Given how much Pentasa and Asacol cost, and given the debate about whether they're even very effective in Crohn's, I personally wouldn't take them if I had to pay full price for them.
In fact, if you have to pay for your own meds, I would recommend azathioprine or 6-mp, with prednisone if needed to start with, since azathioprine or 6-mp take a while to work. That would be a cheap and generally effective way to go.