OK George:
Right now:
Basic Element £1,960
Couple and lone parent element £2,010
Family element £545
Child element £2,780 x2
Total: £10,075
Income: £13,000 - Threshold £6420 = £6,580
Excess £6580 x Withdrawal rate (taper) 41% = £2697.80
Final award: £10,075 - £2697.80 = £7,377.20 (£141.87 pw)
Next year, assuming all values not published stay the same:
Basic Element £1,960
Couple and lone parent element £2,010
Family element £545
Child element £2,780 x2
Total: £10,075
Income: £13,000 - Threshold £3850 = £9,150
Excess £9,150 x Withdrawal rate (taper) 48% = £4392
Final award: £10,075 - £4392 = £5,683 (£109.29 pw)
Overall loss: £1694.20 (£32.58 pw).
However, taking into account NeedaSockAmnesty's point, then perhaps the clearest way to say it is:
If you pay any rent or council tax you may find that for every £1 your income falls, you gain £0.65 HB and £0.20 Council tax relief.
That means that you could gain £21.18 in housing benefit pw and £4.28 in Council tax relief, totalling £25.46. This would lead to an overall loss of £7.12 per week.