Sizes;
Pretty meahingless using letters, they vary way too much by country and even more by who made it.
Best used as an "indication" of approximate size, nothing more.
Production.
Mass produced by the thousands naturally will drive the price down.An individual custom made shirt is not going to be a cheap item.There is alot of work involved.
If you already have a shirt that fits comfortably, take the measurements directly off that and have it reproduced, you'll know its gonna fit.
Material.
Thailand is awash with fabric, there are areas, particularly in Bangkok are designated purely to fabric.You can get nice material and a wide choice, browsing in that area.
Design.
Browse the internet for images of what you actually want to achieve.
You get what you pay for !
No one is going to produce a Custom shirt for under 800 baht thats any good.(generally).
Sometimes its better and more cost effective to buy something you like in a larger size and have it altered to fit you. There are many skilled machinists here who can produce good work, the trick is finding them .????