There really isn't any actual amount stated in the 'immigration rules' that indicate
just how much is considered as 'sufficient'.
Basically, the UKBA will be looking at the amount of money left after any key fixed/regular
expenditure (such as rent or mortgage payments etc)
We all have different key fixed/regular outgoings.
(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 is probably the best guide available.
In principle this means that if it is likely that the amount of money that the applicant and sponsor will have to live on would fall short of that available for a family of similar size from welfare benefits then the ECO may consider this as appropriate grounds for refusal.
The 'Income Support' rates I have managed to find are:-
Type of person....................Weekly amount (2010/2011)
Single - age under 25..............................51.85
Single - 25 or over..................................65.45
Lone Parent - under 18............................51.85
Lone Parent - 18 or over..........................65.45
Couple - both under 18............................51.85
Couple - 1 under 18, 1 aged 18 to 24........51.85
Couple - 1 under 18, 1 25 or over............ 65.45
Couple - both 18 or over........................102.75
Each dependent Child - up to 20 ..............57.57
This was the source for these amounts:-
http://www.focusondisability.org.uk/brates-1.html
Hope this helps