Let’s say you are going to sell your house. You bring in two different real estate agents to value your property. The first agent comes in and inspects your house thoroughly and tells you that based on his expert opinion it is worth £430,000. The next day the second agent calls, she has reviewed the sales of other houses on your street and thinks your property is worth £1.15m.

All valuations are inherently subjective. But half a million quid is a shit-load of subjectivity. You’re now pretty perplexed. Either the first real estate agent is totally incompetent or, alternatively, the second firm and their property survey approach is way off the mark. Or maybe the whole real estate business is a bunch of crap and you’d be better off holding your own auction and letting the market decide.

This is exactly the situation we are in now with respect to brand valuation. I differ from my esteemed Marketing Week co-writer Wally Olins who believes brand valuation is pointless. Brands, like houses, sometimes have to be sold and when they do, a fair value for the equity of the brand must be calculated or we would return to a bygone era where brands remain off the balance sheet and marketers are restricted to fluffy, design-derived roles at the periphery of organisations.

The problem is not whether we should value a brand, but rather where the figure comes from. Most journalists working for the popular press don’t really understand brand valuation so they treat any and all approach with equal attention. Anything with a top 10 brand list will make its way into the media. Look at the success of the hilariously bad Superbrands as an example.

The two most well-regarded brand valuations are provided by Millward Brown’s BrandZ Top 100 and Interbrand’s Best Global Brands list. Both produce the same thing: a ranking of the 100 most valuable brands in the world, and each year we get to see their latest assessments.

The problem for the two companies involved, and marketers in general, is how far apart their annual estimates of brand value tend to be. For example, in 2010 BrandZ estimated the value of the Google brand to be $114bn, making it by far the world’s most valuable brand and suggesting that approximately 75% of the overall market capitalisation of Google can be attributed to its brand value.

In contrast, Interbrand valued Google’s brand at $44bn in 2010, well behind brands like Coke, IBM and Microsoft. That’s more than a minor difference of opinion with BrandZ – that’s $70bn worth of disagreement. Or, to put it in perspective, the combined 2010 brand values of Porsche, Barclays, Audi, VW, HSBC, Ford, Nike, Burberry and Pepsi.

Valuation is a tricky business because, unlike most of the activities and actions associated with marketing, it is expressed in a single, comparable financial number. And like our prospective house buyer, when it comes to the crunch of buying and selling something, the numbers matter.

The vast $70bn difference is not derived from a difference of philosophy – both Millward Brown and Interbrand calculate the value of a brand the same way. They work out a simple net present value based on a calculation of how much money a brand earns, how much of those earnings were generated by the brand itself and the likelihood that the brand will be able to maintain that strength in the years to come.

Both firms also have access to the same financial data for each brand. The difference comes from the method each uses to estimate a brand’s overall strength. BrandZ uses its own internal survey of “2 million consumers in 30 different countries” to assess brand strength. Interbrand relies on its “pool of global experts from over 40 countries” who each complete a 10 item assessment of every brand’s strength.

It’s a classic marketing contrast. BrandZ uses data. Interbrand uses managerial opinion. I know which one I prefer: data beats opinion every time in my book. But the value of different valuation approaches is, itself, subjective. And both firms have legitimate reasons to claim their approach is superior. For BrandZ there is 13 years of extensive panel data and Millward Brown’s track record of research excellence. For Interbrand there is the authenticity of being the first proper brand consulting firm and the only name that most CEOs recognise when it comes to branding.

And we come back to the $70bn elephant in the living room. Like our confused house buyer, smart marketers are now questioning the two firms and the whole business of brand valuation itself. BrandZ has already published its 2011 league table and Google is now valued at $111bn – down 2% on 2010’s valuation and second only to Apple in global brand value terms.

Which leaves us waiting eagerly for Interbrand’s imminent 2011 estimate. Will it close the gap between its valuation and BrandZ’s? And if it does, how will it account for the change? Or will it continue to assess Google’s brand value as less than half that provided by BrandZ? And if so how will it defend such a glaring difference in value?