Microsoft has accompanied the third of its anti-Apple commercials with a creative document detailing what it calls the ‘Apple Tax’.

The latest ad follows the same theme: ‘ordinary’ shoppers (this time a mother and son) head out with a $1,500 budget and wind up choosing a PC over a Mac. This time the requirements are for a fast computer suitable for gaming, though the child is swayed to a PC laptop because of its Blu-ray capabilities. As usual, the Mac is dismissed as pretty but inadequate and expensive. Here is the ad:

With the shock value of the direct attack gone, online reaction has been somewhat muted. It seems that for future ads to have much impact, Microsoft needs to tweak the concept, perhaps getting much more specific about why a particular PC is better than a particular Mac.

Meanwhile Microsoft has taken the concept of the Apple Tax, a line created by the firm’s consumer marketing chief in an interview last October and turned it into a very literal marketing program. The firm has put out an 11-page study claiming that an average family buying two PCs rather than two Macs would save $3,367 over the following five years, and even produced a dummy IRS form itemising the costs.

Only around half the supposed savings come from the purchase price of the machines. The rest come from software, support and various upgrades. However, several points are either confusing or plain misleading. For example, when it comes to office software the study assumes the buyer already has a copy of Microsoft Office from owning a PC but would have to pay to buy Mac equivalents.

In other words, it’s a comparison of the cost of switching from a PC to a Mac, not a straight comparison starting from scratch. That may be a realistic scenario for many would-be Mac buyers, but it’s certainly not fair: after all, a Mac user switching to a PC would face a similar ‘Microsoft Tax’ on software.

The figures also include costs for the MobileMe service and in-store Apple care, but don’t take into account PC equivalents or free alternatives for Mac users.

Ironically this appears to be one of the rare situations where added detail makes a much less convincing case. Close study of the ‘Apple Tax’ reports leave you with the feeling that Microsoft is greatly exaggerating the genuine price differences. The TV spots, however simplified and manipulated, at least make a more credible general point that Macs are usually more expensive.