My two-cents worth. Asking for a discount (including here in Oz) is always acceptable (but always with a grin and a laugh). That said, there are Thai places where you bargain and others where it is a waste of time.
Thai sellers have learned that foreign tourists are used to paying fixed prices in their home country. And the Thai are quick to copy. So, in department stores and similar, you will be asked for a fixed price (even if it is 'outrageous') and if you want the product, you pay. In other venues you can bargain the price.
Question time: When you travel, don't you seek out the lowest airline price or the 4-star hotel that is willing to be bargained down to 50%. Most of us do it all of the time in one way or another. It largely depends on whether you enjoy asking for and getting a discount. At home I usually try-on for a "seniors discount" - sometimes I win, sometimes not - haircuts, lunches, apparel, anywhere I feel there is a chance.
Then I go to Thailand or South East Asia where bargaining is a way of life and it is a fun-game that sellers and buyers regularly play. (That's one of the reasons that every seller I know has a calculator handy. <grin> ) If you bargain you may get somewhere near the correct retail price and the seller still makes a good profit. If you don't bargain, the seller laughs all the way to the bank - "silly farang, he pay too much".
Normally, I would not buy "silk" products in Bangkok or any of the other major cities. Last November, some friends and I were touring in the Golden Triangle. In a small road-side market out in "nowhere" we were attracted by a thermal fountain. So we stopped for lunch and took a walk around. We found some beautiful silk shirts (even silk on the roll). The aficionado among us declared the silk "real", but the first asking price was a turn-off. So I started to play the bargaining game. ":Oiy. Expensiiivvve. I poor <grin>" etc etc (having just driven up in a huge 4WD Pajaero). The final price was 30-40% of the first asking prices (depending on the size of the garment) - they were happy to sell 10-12 shirts and we were happy with the price that we paid. In the meantime, buyers and sellers had a lot of fun and laughs.