Check out this great article I stumbled upon:

The truth behind 0% interest promos

The Canon 400D camera cost P48,500. Its interest free if you pay for 12 months. The monthly payment is P4,042. Cut down to monthly bite-sized pieces, it doesn’t look too painful. Then I asked the clerk, “What if I don’t want to pay by credit card? I want to pay in cash.”

“Mam, its P44,950 if you want to pay in cash,” she says.

Technically, there’s no violation of Administrative Order 10 issued by the Department of Trade and Industry, which put a stop to the practice of putting a card price and cash price in one price tag. But if you pester the clerk in a nice manner, he will tell you that he can give you a discount if you will pay in cash.

So, is the 0% interest for real? MoneySmarts concludes the nice round “oh” is just a figure, often in red, to catch people’s attention. The interest has been included in the price already. If I pay P4,042 a month for that Canon 400D, I will be paying 12% interest on this installment promo and not 0%. If I pay in cash, I would save P3,550 – that’s almost 2 gigabytes of additional memory for the camera.

Source: Money Smarts – Inquirer.net blogs