There are no cheap gundams in JB, at least to my knowledge. You're better off getting them from KL, or buy them from forumers around here. But if you are desperate, here goes:

Places to get gunpla in JB:
1. Dunno what name toy shop in City Square, 3rd floor, on the way towards the bridge linking the customs building. You won't miss it. Lots of new stuff like 00 and SD Sangokun
2. Another toy shop in City Square, 4th floor... lots of japanese anime stuff and candy there too... again you won't miss it, it has bright orange walls.
3. Pelangi Leisure Mall comic book store, 2nd floor besides the camera shop - lots of comics, decent stock of gunpla, limited (but at least present)tools, paint and accessories, absolutely crazy prices.
4. 无名comic book store, 4th (or 5th?) floor Holiday Plaza - again, crazy prices
5. Skudai parade - dunno where I can't help you with this place since I've never been there before... myky should have more details here
6. Your friendly neighbourhood Jaya Jusco or Parkson - not many choices, prices are somewhat crazy too; but sometimes during sales you'll manage to find some decent priced gunpla... Last year Parkson had 1/144 HG Exia going for approx RM40+ after discounts. That's probably the best you can get in JB.

There might be a toy shop or departmental store in Danga City Mall selling gunpla too. My friend spotted it there and notified me last week, but I haven't got the time to check it out. Don't think it'll be any cheaper anyway, so unless you're dying for gunpla you can forget it.

Hope it helps, but I don't think you'll want to pay extra... go to LYN's garage sales instead.