Go see Bobby at Raja's Fashions. He was recommended to me by business partners in town, and largely serves the expat community. There are a couple of "Raja's" in the area that all try to trade on the same brand. Be sure to go to the one near Nana Plaza (Soi 4 off of Sukhumvit Rd, near the JW Marriott Hotel).
Bobby is great. There are probably cheaper in Bangkok, but I got custom-tailored business shirts for around $30 each, and a tailored suit for under $300. I'm not an expert, but the quality of everything is good, and was complemented by another tailor in HK a few months later. I've been to Bobby a few times, and will keep picking up a few items from him every time I go back.
I must agree with the Rajawongse recomendation
At this point all my clothing from Formal wear to Business carry his label
They also make an incredibly priced Cashmere top coat
Jesse and Victor know how to make a quality product.
In fact one time I was going to Bangkok and called my wife to ask if she packed my slacks because I know I hadn't.
Well neither of us had packed them.
I sent Jesse a e-mail from the chicago airport and new slacks were in my roomwhen I got to the hotel.
They are simply fantastic