Shoppers mulling whether or not to buy a new iPhone better strike soon or else face paying almost $20 a month more.

A whole bunch of customers come off contract when an iPhone launches. This represents a big opportunity for carriers, not only in resigning existing customers, but in stealing customers from rivalling telcos.

Customers end up being the real winners, with discounted plan pricing and bonus data inclusions, only the rewards can be reaped for a short time.

Source: WhistleOutSource: WhistleOutSource: WhistleOutSource: WhistleOut

History reveals iPhones are cheaper from launch day up until Christmas. Prices for an Apple iPhone can increase by up to $19 a month more -- or $456 more over a 24 month plan -- according to research compiled by plan comparison site WhistleOut.

Prices for the models with greater storage capacities increased most, such as the 64GB and 128GB variants.

“Typically, you’d expect the price of phones and the amount that customers pay within or above their contract price, to fall over time. The demand for the iPhone shows a completely different price trend to what we all expect in electronics,” said Nick Rumpff, General Manager of WhistleOut.

The revelation is all the more pertinent this year as pricing for Apple’s 6s series has increased. Outright prices across the board are almost 25 per cent more expensive, depending on the model.The below graphs illustrate pricing hikes for the iPhone 6 and 6 Plus.

Read more:Apple iPhone 6s review: Experiencing the coveted third dimension

Latest news and slideshows on Apple