Good luck in getting an "off the rack" suit... or shirt... in Singapore. Even without size considerations, the Asian body is different than the Caucasian body, and you will be hard pressed to find something that works. If you want to try this route, that would be the major department stores... Isitan, Takashimaya, maybe Robinsons... maybe Marks Spencer.
For custom made: Dodgy name - Christian Armani - in the Far East Plaza shopping center at 14 Scotts Road. Excellent fitting and excellent tailoring. I've tried four different places... none are as good as this one. Ask for Dave. You could offer to buy me a beer and see some of their work in person.
As for costs - shirts made of teflon cotton will come in around S$120, $30 to $40 less for cheaper fabric. Also depends on "customization"... I have different colors for the insides of colors and cuffs. You might get as low as S$60 for single fabric color and lower end fabric. But... what the hell... if you are going custom, why get a shirt that looks like you bought it at Walmart?
Suit prices are really dependent upon the fabric. Expect to pay between $400 and $900, depending upon fabric, for one suit jacket and two pair of pants. More if you want the vest.
You can find cheaper outfits than Christian Armani... they do their own cutting and sewing... others farm the work out to Batam... harder to control quality. And I have to tell you... I really don't like the Indian tailors... mainly because if you are a white expat, it is impossible to walk past a shop without a, "hello sir, may I interest you in a suit?" I just refuse to deal with someone who has seen me every two weeks for three years (that's where I get my hair cut) and still asks me this asinine question every time I walk by.
I'd opt for the more expensive route. Better fabrics hang better and look better... and last longer. I paid $600 for my suits... they look good after five years... and I can stuff them in a suitcase, fly to Melbourne, hang them up, and in an hour or two, no wrinkles... ditto my shirts... telflon cotton are excellent if you travel... minimal wrinkles.