There really isn't any actual amount stated in the 'immigration rules' that indicate just how much is considered as 'sufficient'.
It's up to the sponsor and applicant to prove there is sufficient for them to maintain themselves.
Basically, the UKBA will be looking at the amount of money left after any regular non-discretionary expenditure (such as rent or mortgage payments, council tax and secured loans etc)
Everyone has have different non-discretionary expenditure.(eg payments of Child support would also be considered a key fixed/regular payment)
Various Immigration Tribunal's have concluded that it would not be appropriate to have immigrant families existing on resources that were less than the 'Income Support Level' for a British family of the same size. This then is the best guide available and is one of the tools used by UKBA
In principle this means that if it is more likely than not that the total amount of money that the applicant and sponsor will have to live on will be below what the income support level would be for a British family of that size, then it may be appropriate to consider refusal of the application on maintenance and accommodation grounds.
The 'Income Support' rates for tax year 2011/2012 are £105.95 per week for a couple and £62.33 pounds per week for each dependent child.
ECO's are not accountants but, if you are generally living within your means and can afford the cost of another adult consumer joining the household, this should be immediately obvious from the supporting financial evidence (bank statements/payslips etc)
Only those people with a discretionary amount of money close or equal to the equivalent of means-tested benefit might feel the need to make a budget plan to signpost how they can afford another consuming person in the house.
Hope this helps.