As of 2015, prices for headphone foam pads range from under $3 to over $30 depending on the make and model of headphones. Some headphones do not have replaceable pads, so it's important to check with the manufacturer to see if the foam pads are removable and replaceable.
For some headphone models, it is possible to purchase foam pads not made by the manufacturer. This can give a more comfortable experience, or even a different sound. For example, Steve Guttenberg from CNET notices a dramatic increase in comfort when using Brainwavz HM5 Replacement Memory Foam Earpads in his Audiotechnica ATH MSR7 headphones. He also notes an expansion in sound stage. When buying aftermarket pads, be sure to check the shape and diameter before purchasing.