I don't think that is correct. The Duchy generated 19 million pounds last year and had around 11 million pounds in official expenses, so the net was 8 million pounds. Charles paid about 4 million pounds in taxes.
However, if the Duchy were incorporated and paid a 50% tax rate (I think the UK corporate tax rate is actually lower), the remainder would be 4 million pounds. If Charles still paid the 50% rate he would have 2 million pounds left over.
People incorporate businesses and pay more taxes when there are benefits to doing so. For example, many businesses incorporate because it shields the shareholders from personal liability for the corporation's debts. But there is no reason for the heir to the throne to do that. The papers should also note that he gives large amounts of money to charities.
Another note: Charles has always voluntarily paid taxes, but I think he reduced his taxes to 25% when he married Diana (he took a tax deduction), but he increased his payments to 50% when it became an issue in 1992.