In your calculations, have you factored in that being a SAHM is not free? You may think some of these costs I mention are tiny but they are all there and if you make no account of them at all, you will start to find it.
Essentials are:
More heating and lighting costs
House gets more beaten up
Consumables such as loo roll get wasted, however vigilant you are.
Phone calls etc to arrange playdates or whatever.
Then you need activities for DC, which can be totally free, but it makes for a very stressful life if you never want to spend money. In my SAHM operating budget I would allow for:
Toddler group fees -roughly £1 a time
Soft play entry
Swimming
Bus fares / petrol / bit of each
Craft materials and sand for sandpit for fake hols!
Coffee etc. with other mothers (very hard if you do without this)
Costs will increase in school hols when the cheap toddler group stops and you have to shell out more on expensive activities
You will also require contingency to cover things that may arise:
Household breakages (I am superbly vigilant [modesty emoticon] but DS did knacker the washing machine once doing something I htought was harmless)
Being locked out
You being incapacitated and requiring emergency childcare/DH to take unpaid leave.
I am very lucky to be able to SAH and feel relatively free to make these expenditures. I am supremely grateful to DH for this. It would be a far more stressful role if I were confined to the park every day and one toddler group a week. I do lots of craft out of scrap and make my own playdough for pennies but you still need glue, glitter etc.
If you're slicing the household budget to the bone to allow you to SAH you need to be aware of these hidden costs. And I do think you are contemplating making cuts that you and DH will really feel, so you BOTH need to be on board.