Shopping in Singapore can be a bit hit or miss. Many items can be found a lot cheaper in cities like London or Hong Kong. But if you know where to look there are definitely bargains to be had on certain kinds of items (but not cameras. NEVER BUY A CAMERA IN SINGAPORE!).
One such item is G Shock and Baby G watches from Casio. While the malls of Orchard Road are full of stockists, you want to head just a little further out of that well known shopping area, to another hotel and shopping mall complex The Bencoolen, not far from the Bugis markets.
Here you can find 2 or 3 stores selling a wide range of models new and old (in our experience, usually including the latest models, but your mileage may vary), look for them on the street level shops near the west entrance. Prices really are good, something like 30% less than anywhere else in Singapore, and probably cheaper than they are in your home country too.