ADDENDUM: In a completely different context (namely the UK), here is another attempt at putting a specific cost on a basic income scheme (from people who are favorable to it). Based on a similar multiplication, they arrive at a total cost of GBP 276 billion and then explain how to pay for it.
The main difference with the proposal discussed above is that this citizen's income would be GBP 71 per week for an adult, well shy of the UK's poverty line. (Note however that poverty is defined and measured differently in Europe and that it make most sense to look at households rather than individual persons). It can be valuable nonetheless but it's a very different idea.