It's the amount after after rent and council tax only and not any other bills (although if you like you can include a budget detailing all your expenses).
UK couples on government income support receive £102.75 per week to live off after rent and council tax have been accounted for. So you need to show you have at least £102.75 per week of your own money after you have paid rent and council tax (but no other expenses or bills), as that is the amount you would receive if you were allowed to claim UK income support.
With just the income from his job, if he brings home £800 per month:
£800 - £310 = £490 after rent and council tax.
£490/4.3 (number of weeks in a month) = £113.95 per week
and if he brings home £900 per month:
£900 - £310 = £590 after rent and council tax
£590/4.3 = £137.21 per week
So, in either case, you still have more than £102.75 left each week and so you will meet the financial requirement for the visa.
However, you can supplement this income with any savings that either of you have and also if you do secure a job offer before you apply for the visa, then a letter showing the offer of employment and your salary can also be used as part of your financial evidence.