I notice my question hasn't actually been effectively answered, since the answer provided is unacceptable and factually incorrect.
This is pretty much the most important question that the Cape Party needs to answer before any other issues can be settled, and if they can't be honest and upfront about this very basic question then they deserve to be ridiculed for the joke that they are.
Your question has been answered. That you refuse to recognise the answer is YOUR problem.
We have outlined a territory (which is framed by the constitution) which we can claim as our own. You ignore the fact that the Cape Colony was expanded by the same British empire that forced a Union of South Africa. You also identify that colony at its largest extent, even though it only briefly consisted of many areas in the Eastern Cape and North West.The Cape Colony existed for 258 years before the Union, and for 154 years before the arrival of the imperial british, who forced an unnatural expansion.
The Cape's natural borders are clearly marked along the outline of the contiguous municipalities mentioned before. They mark an ancient border of the Khoisan resistance against bantu migration. The Khoisan (those that survive) have largely taken on mixed race status and adopted Afrikaans as their language.
You previously have mentioned that there are assets which "belong" to the national government and the SA tax payer. This is a rather warped take on how things stand. The South African government owns nothing. It administers the land included in its territory on behalf of the people, in whom ultimate soveriegnity lies. The people have the power to dissolve the social contract at any time they wish, and to decide on other social contracts (eg one that involves the dissolution of the Republic of South Africa).
Nothing will be owed to anyone should the Cape secede. You make a further mistake, in that you forget that South Africa seceded from Great Britain (who had previously owned vast amounts of territory, government buildings, military installations etc) and paid no such money, as the example you talk about. The territory was simply given to the South African government as part of its treatise toward sovereign independence. South Africa was not a soverign nation until 1927, when it took control of its own foreign policy and got a flag. In 1931, after the Balfour Declaration, South Africa became a dominion with the British Crown represented in all but name, under the Governor- General, who took his advice from Parliament. He was a puppet/symbolic Chief Executive. South Africa completed its total secession from Britain in the 1950's and culminated in 1961 with the Declaration of a Republic. We did not pay a cent to Britain for what "They owned". Nor did we ask the permission of the British people.
Neither did the US, when it seceded from Great Britain. There too, the British "owned" territory, land, buildings, military installations. No one paid a cent to anybody.