It's easy to be cynical about the tailor shops on Sukhumvit area, if you have had a bad experience.
Ultimately you do get what you pay for, but it should be reasonable prices for well-tailored shirts and suits made from quality materials.
I highly recommend "Andrew and Walker" tailors - You can have a conversation with the tailor while he's measuring you up. Discuss what style you're looking for, help you choose the material (not force it on you).. etc. A very reassuring process. The final product is excellent high quality.
They are inside the Lotus Hotel Soi Sukhumvit 33.