We don't know what the failure rate of the Falcon will be, whereas we do know what the failure rate of a Proton is (~10%). Satellites are expensive and delivery is not guaranteed, so every launch provider always gives a discount to early customers who put their payload on one of the first few launches of a new system.
Pvarangot's point was that the price for booking a launch today is almost certainly less than, say, booking a launch after Falcon has had ten successful launches on its first ten attempts. If Falcon has Delta's failure rate, then SpaceX can charge a premium. If Falcon's failure rate is closer to Proton, they will have to underbid the Russians to get customers. We won't know until Falcon has a few launches under its belt. Saying they are booking launches at guaranteed prices today is true, but only gives a hint to what it will cost customers five years from now.