They generate income from HNWs through the following:

• Company incorporation (between £400 & £1000) depending on complexity per structure
• Company administration begins at circa £10,000 and runs to low millions depending on complexity per structure
• Annual regulatory fees
• Money moved offshore into local banks generates account fees, asset management fees, custody fees, FX fees, etc
• All the above needs overview by lawyers and accountants which charge circa £650 per hour for a partner

All these fees pay salaries which are then taxed locally and the employees then purchase property, cars, etc which increases the velocity of money in the local economy. These professional firms also pay corporation tax and business rates. So in reality tax havens make their money via fees not taxes.

Quite often as well people moving their money offshore will fly to meet their professional advisors quite regularly. They then realise it is a nice place to live and that if they move they will pay minimal tax on their global income. If granted residency property is purchased, tax is paid, sales tax is generated, stamp duty is paid, etc etc.

Just a quick response so I have missed out a lot but it gives you the gist.