A meal in an average restaurant would cost you about SGD 15 - 30, of cuz depending on where you eat and what you eat...but if you go cheap, you could go to food court and have a meal that would average SGD 7 - 12. Also, if there are someone who take you around, there are some hawker food which is very good, and relatively cheaper.
Shopping wise would be similar to HK's, but there is a 3% GST(Goods and Service Tax) on top of all goods, so there could be the difference
For souvenir, most people will buy 牛肉乾,肉鬆.Some will also buy different type of 藥油. Explore around, you shall see something interesting. ^_^