In my experience, there's certainly some truth to the Apple tax, especially here in New Zealand where Apple products are sold through third party distributors who want to add their margins to the retail price. Dell and HP will both sell directly to you, which can result in their prices being lower.

In saying that, what often gets overlooked is the "Apple rebate" that you get when you come to sell the machine. This would be difficult to measure with any degree of accuracy, but in my experience Apple products tend to hold their value for longer than Windows machines.