How about 'helping' to create one?
I think I want to have a go at this, and although it seems a massive task, it shouldn't be that difficult to get in the rough area. And I'm sure there'll be plenty of people willing to correct my figures/method as soon as I go wrong
So, here's v1.0 of the method:
1. Sum up all benefits, payments, grants, subsidies, that are paid out by the state to private individuals
2. Find out the total of all taxes currently collected by the state
3. Investigate the cost of administrating the benefits from point 1 and sum it
4. Look into the 'tax gap' - the difference between the amount of tax that should be collected but isn't, due to fraud, corruption, etc
5. Look at the previous studies on basic incomes and their effect on working hours, and therefore tax takings. Apply this reduction to our total from point 2
6. Look at previous studies on basic incomes and their effect on public service demands, such as hospital visits. Use these figures to look at our spending on public services and add the projected savings to the sum from point 1. (Not everything will be a saving - education will be in higher demand so there will be costs rather than savings here)
7. Try to find the highest possible amount of benefits somebody could be in receipt of (most expensive part of London, on HB, multiple kids, etc, etc) as well as the likelihood someone will be on this. Find the average amount of benefits received. Find the lowest. Graph it.
8. Use the graph from 7 to calculate how much the universal basic income would need to be set at to cover 90%, 95% and 99.5% of the population.
9. Use the calculations above to see what we could afford to pay using the current budget/tax regime. How far off is it from the numbers generated in point 8?
Those things are all fairly doable, even as an economic dunce, I reckon. Nothing in there is difficult to calculate, and there's not too many assumptions. The difficult bit will be point 7 I reckon, finding the average amount of benefits paid out.
Once you've calculated those, then we could play around with tax rates and see what that does to the figures, but then you're getting into much harder to predict territory.
Any glaring omissions/objections so far?