I've only recently arrived in Jakarta, but some of my friends have used Mumbay Tekstil in Plaza Mayestik for suits, shirts, and trousers. I gave them a shirt order and am currently awaiting the results. I believe the cost for the suits have been less than those quoted above for Kwong Tung, but this might be a fabric issue.
The fabric selection seemed decent but, frankly, I'm a novice in this area and don't know what to look for.
Does anyone have any advice on general websites or resources on the Jakarta market? Perhaps this forum can serve in that capacity.