While Jim Thompson has a well-earned reputation for offering the finest Thai silk, such quality doesn't come cheap. You will easily pay three times more for silk items carrying the Jim Thompson logo than you will for similar items from other shops, although of course the quality may not be the same. Shop around a bit to help you decide if the quality is worth the premium price.
It's also worth noting that Jim Thompson is not a tailor shop. While the main branch does have some dress-making capabilities, this is not the place to go if you want a suit or other clothes tailor made. It's strictly off-the-rack at Jim Thompson.