Rogers Iphone 3GS plan. 99 Dollars a month is that cheap?

I really want to buy the Iphone 3Gs however on the ad it said it is $99 a month for a 3 year contract. What does that mean? Meaning you pay $99 every month + your bill for 3 years. Or you pay $99 once and for the rest of the 3 years you only pay your bill which will be $40 dollars because I'm interested in a student plan.

Please help me.. I really want an Iphone because I want to buy and Itouch and my cellphone contact is ending next month.. so to save money I'll just buy an Iphone.. :\ .. unless it's expensive.

This is the ad: http://www.rogers.com/web/link/wirelessBuyFlow?forwardTo=PhoneThenPlan&productType=normal&productId_Detailed=3GS8BLK&N=52+11+4294945848