I suspect that the declared tax revenues of the financial sector are just a small part of the picture. It is likely that much of the enabling infrastructure, like agencies that register companies, nominally belong to the "legal" sector, and perhaps to avoid detection, some others may be misclassified as well. Then, of course, there are lawyers. And since many companies are used as flow-through vehicles solely for money laundering, these can be counted as part of the shadow financial sector as well; while the taxes they pay are not high, they still pay some kind of taxes (say, in single digits?), and the sheer volume makes it worthwhile.

If I were to assess the actual share, I would go by elimination: everything sans tourism, minuscule local food production industry, and transportation.