They won't give you the details, i.e. the amount of discount via email or even over the phone.
Just take your used printer to Harbour Front Tower 1, Canon Service Centre (4th floor) and mention that you're there for the Privilege Purchase Programme (not trade-in). Tell them the model that you want, they'll offer you a price and once the deal is done, they'll take over your old printer (no need cables, accessories or cartridges) and place the order for you through their dealers.
I was offered S$162.64 (SRP is S$199) for the Pixma IP4870 ...