I honestly think having another child cannot be based solely (or even mainly) on how much money you make. Children can be expensive, but honestly, the things that make them so expensive are food, clothing, childcare, and healthcare - and you can make smart choices with all of those. Your health insurance costs are unlikely to go up because of an additional child - you want to check to make sure, but usually it doesn't matter how many children you have, the cost to insure is the same. You can negotiate with your doctor on the co-payments/deductibles. Food - if you breast-feed, that's free. When the baby's older, he/she can eat what you eat - no need for jarred food. Clothes - you have two children now, so the baby can wear the clothes they have outgrown - even if it's a boy, he won't care if he's wearing pink! Childcare - since your mother would not be willing to care for a third child (and I wonder if that's to "prevent" you from having another), you would have to consider that. Could you do some work at home that will equal what you are paid part-time, so you can care for the kids without outside childcare?
Your mother's concerns are legitimate, but frankly, those concerns are there with OR without a 3rd child! They are not going to appear OR disappear with the child's birth. Your husband may want to keep careful track of your expenses, and see what you can cut back on to create a savings account, but as I said, that would need to be done whether you have another child or not. As you WELL know, having even one child requires sacrifice, and having more requires more sacrifice, but consider the "pay-back"!
If you and your husband really want to have a third, even without the savings, etc. I say go for it. The joy with more than make up for the sacrifices you will make!