Not much. For a country the size of the United Kingdom, for instance, with 64 million people, we would need as little as £5 per person for the initial phase, which would total £285 million per month.
CalculationFollowing calculation has been done for the UK:
64 million (UK population)
- 3.5 million (under age of 9)
- 3.2 million (mentally not capable)
= ~ 57 million people
57 million people * £5 for each person = £ 285 million (per month)
For one year, that is £ 3.4 billion or 0.13% of GDP.
For the year 2014-2015, the projected tax revenue in the UK was £648.1 billion; therefore, for the Basic Tax Control, we would need to set aside 0.5% of total collected taxes. This is miniscule, especially if we consider the good things that BTC can accomplish. Think about one more number: the UK's military expenditure for the year 2014 was £60.5 billion.
Now, ask yourself: if we can find money for wars, can we find 20 times less money to create peace and welfare for everyone?
If you still think that neither of these things is an option, think about one more number — one important number that must not be forgotten: the amount of tax avoidance. In the UK, just for the tax year 2012/2013, the price tag of tax avoidance damage was £34bn (10 times of amount we need for BTC). It is a huge number and also the reason why it is important that everyone pays a fair amount of taxes. Movements like "Fair Tax Town" have brought this issue to national attention, especially bringing to attention that tax evasion is available for any business, and it is very easy to setup.
For Basic Tax Control, it would be possible to take the money without any significant pressure to other services, but, if we could retrieve money dodged by big corporations, we could get significantly more money — more than enough to do the trial test of a Basic Tax Control system.
Basic Tax Control should be funded from collected taxes, but, for the early implementation and test runs, it would be possible to use private (our own) money, but it should not be continued as a practice. The end goal is to take money from already existing means of socially-collected funds: taxes we already collect for public purpose.
Should the Basic Tax Control amount always be the same or should it change over time?
Every country or community (if privately funded) should decide the amount of money they want to dedicate for BTC, but, whatever amount is decided, it must be the same for all members of the group.
The BTC amount per person should not be fixed. Potentially, it should increase over time, until it gets to the proportion that is equally comfortable for the government and for the public. The more directly democratic society is, the more taxes will be controlled by it. The idea is to shift/disperse a portion of responsibility from the government to each person in society.
Increasing the funds controlled by the people should happen gradually. For instance, if the trial has been successful after 3 months or so, the amount should be increased relative to the size and amount of success. Ideally, the increase should be calculated in a way that would achieve optimum efficiency for the next step.
In theory, the public could control 100% of the tax revenue, but that would require a high level of understanding about public services and public needs, like welfare, healthcare, state pension, defence, justice, etc. Any disproportional funding could cause disruptions and issues. Although 100% is very difficult to achieve, it is possible with the proper level of knowledge of people who are involved.