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.
There are several "factory outlet" stores operated by Jim Thompson, where you might be able to find clearance items at special prices. These outlets also have a huge inventory of fabrics for wall coverings, upholstery and clothing.