Peninsula expensive?! Firstly, it's not authentic. Secondly, 15/18 bucks is the price they tell you, but they can knock off a couple of bucks if you bargain. If you're a regular customer the shop owners will sell it to you for about 12 bucks each and if you buy more than 1, they can go down to 8-10 bucks.
The prices of band tees are usually 20+ and 30-40 bucks for tour/original tees.
If you're wondering what's the difference between the imitation and the authentic t shirt, it's the quality of the t shirt that they print on. Of course, this varies due to the different suppliers that are in the market. I'm not saying that all authentic band t shirts print on good/better quality t shirts, but most of them do.
You can get the authentic band t shirts at Image 2001 in Far East Plaza but their prices are pretty jacked up. The best bet would be to buy it online from sites like merchnow or the band's official merchandise store. Just do a Google search for " -Band name- merch store". Do note that not all sites ship to Singapore but there's always Vpost for that.