I wish it were law that parties making pledges actually had to have a 'How we intend to fund this' part of their manifesto with actual breakdowns.
£## billion from x will be used to fund y.
At the moment everyone is promising the earth but pulling the earth out of their arse.
I know it is so that they don't have to be held to things from their manifesto, but that is the entire point of a manifesto. I really don't feel I can make an informed choice at all.
No party can accurately predict x or y; particularly x. Government revenue comes from tax receipts. How much tax is made depends on the tax rate, but also how much income people are earning and how much capital people own. Those last two things vary very strongly with the economy. When the economy is bad, the government gets less in tax receipts, when the economy is good, the government gets more in tax receipts. You can guesstimate with different degrees of accuracy, but that's about it.
People give economists a lot of flak for getting things wrong, but I think that's unfair. When you make a prediction, you need two things: you need a theory, and you need observations. When you take the observations and apply the theory to them, you get a prediction. Increasingly, the trouble is less that the theories are wrong, and more that the observations are wrong, or just simply missing. You couldn't predict the 2008 recession unless you had a fairly intimate knowledge of collateral debt obligation risk security; and this knowledge is something that the only people who knew it (financial institutions) had a big reason not to impart.
In hindsight, once you have this observation, you can plug it into the theories and they give the right output about what would happen in 2008 and beyond - a collapse in liquidity leading to recession. But we only really have these observations in hindsight. When economists predict recessions, we probably won't have them; institutions make the necessary changes to avert them (I mean, they'd be useless otherwise). If they don't predict recessions, you're fine as long as all of they have an accurate picture of all the relevant variables. But that's very difficult, and that makes predictions very difficult, and that makes costing governments very difficult indeed.