According to Forbes Magazine itself, in their article The Forbes Fictional 15, they say:
Net worth estimates are based on an analysis of the fictional character's source material, and where possible, valued against known real-world commodity and share price movements. All figures are as of market close, April 1, 2011.
And in the page devoted to Lucius Malfoy (also linked to in the question), they state
With the price of gold at record levels, stockpiles of gold galleons have appreciated significantly.
So Forbes is basing their estimates on stockpiled gold. But this could create an issue with their figures if they're basing it on the value of the gold in magical currency, as opposed to basing it on the approximate exchange rate (1 Galleon = £5.00).
A careful reading of the Harry Potter Wiki article on Lucius Malfoy reveals no mention of where the family wealth came from, whether it was property or land or investments, acting as loansharks, or extortion. There is mention of wealth, but no indication of any limits to their wealth or of the source of the wealth.
(And studies have shown, within the past few years, that, in the muggle world, once one attains a net wealth of $70 million, that additional wealth doesn't tend to show any effect on one's lifestyle, so there's really nothing that shows the Malfoy fortune is much above this $70 million watermark.)
So Forbes is basing their answer on an assumption that he has a large stash of gold, but there's no documentation to support that.
My personal guess would be that their family gained their wealth through shady or illegal means, but that's as much as a S.W.A.G. as Forbes referring to his stash of gold.