While it's predictable that a lot of wealth was hidden, a lot of the success of organized crime is actually not quantifiable in monetary terms. I would say those values are correct.

Criminality, in Sicily as elsewhere, exists within a social context. For much of its post-unification history, the social context of Southern Italy was one of very weak state action. In a place with relatively weak state on top of little material wealth, many of the advantages of organized crime were in fact tied to expressions of power more so than wealth in monetary terms.

The origins of organized crime in Italy, while still somewhat contentions, are often attributed to weak institutional guarantees allowing large and medium landowners to exploit, through means of physical and social coercion, both their own renters and neighboring small landholders. While this phenomenon was well underway prior to unification, the policies of the unified Italian liberal democracy only strengthened the position of these landowners thanks to their newfound political enfranchisement and the reliance of the Italian state on this class of southern landholders for the south's own administration.

By the late 1980s, the characteristics of southern organized crime had undergone a two-step change. The first, earlier change, was a stratification of the south's own middle class: one more urbanized group derived most of their wealth from work in the public sector, professionalization, and diversification into real estate (the natural progression from land ownership). While many still owned land, this was no longer their defining source of income. The second strata, who lived farther from urban areas and would remain comparatively much poorer than their urban counterparts, continued to derive their income from agricultural land ownership. While the two strata could and did intersect often, it would be this second stratum that would garner the most attention on the second step of fundamental change, which occurred as a consequence on the one hand as progress in technology eased communications and allowed for the rest of the country to not only become aware of social conditions in the deep south, but also allowed groups in the deep south to perform more more complex criminal acts, but also an expansion of state action into the Sicilian interior doing pretty much the same thing via a different mechanism (this, more generally, is a pattern exhibited in the whole of the deep south: Calabrese shepherds shooting each other over blood feuds only began holding up cars when the highway authority paved their roads making them passable to automobiles, and thus was born the Calabrese Mafia).

While the phenomenon of coercion and exploitation by landowners in remote corners of the Sicilian provinces continued to exist right up to the late 90s and beyond (sharecropping was merely replaced by labor violations) some did indeed choose to move into new illegal activities and they could certainly partake in high-risk and high-reward activities which include trafficking humans, prostitution, and buying and selling illicit goods. Criminal cells originating from Sicily existed in Northern Italian cities since at least the 1970s, and from there spread to the rest of the world. But wealth, in the trade of illicit goods, had a precise monetary value only when the goods are materially sold. So just as an example, drugs not only need to be procured, but they need to be delivered to a dealer before they can be monetized. The cash turnover is then typically very rapid, as the next shipment needs to be paid for. Thus while in theory the aforementioned process is a turnover of millions of Lira, in material terms the only visible result might be a few thousand lira stored in a roadside pizzeria's cash register, taken out and moved elsewhere when there is a feeling the local Magistratura is getting suspicious. Thus it makes sense that a lot of the value of organized criminals' wealth is either hidden or tied up in illicit goods, paling in comparison to whatever is able to be confiscated upon their arrest and conviction.

But in the tradition of the old 19th century landholders, the true currency of organized crime is power. What is the value of controlling a sleepy Sicilian fishing harbor through which your illicit goods transit? What is the value of having a credit line with every shopkeeper in town? What is the value of guaranteeing that a local politician can get reelected? Organized crime's power allows for its leaders to live comfortable lives, even if they are not extremely wealthy in strictly material terms.

As a final thought, one thing that you might want to consider is purchasing power parity in Italy. In the 1980s and early 1990s, the Italian Lira had been devalued with regards to the US Dollar. But the inflation rate, while high for an OECD country, remained fairly constant at around 5%. This means that the basket of goods purchasable with a given amount of Lira remained fairly constant, but that same basket purchased with US Dollars converted into Lira was getting cheaper. So it would be more useful to compare the criminals' income confiscated to income earned by ordinary people in the same area. Be they in agricultural communities or urban communities, rest assured that organized criminals were doing well by the standards of the communities they were part of; while they interacted and developed a relationships with the wealthier urban bourgeoisie .

My sources for organized crime investigations are the works of Roberto Saviano. However, Saviano is not a historian, but rather a journalist. I don't know if a strictly historical work has been written on organized crime in Italy, however more recent works by Paul Ginsborg do touch upon the issues tied to organized crime and (especially) its intersection with politics.