50% of your income goes to needs. So take the average cost of a 2 bedroom apartment, food, internet, phone service, transportation, minimal clothing allowance, healthcare and education cost.
Ask them if they follow this rule. They probably will. The next step is to bring up the number and how you got to it. Ask if the company was profitable last year and by how much. It's best to bring this information with you to show that they can pay you your asking price.
If you're going to be full time, 40 hours a week, that means your work needs to be paying you that number x2 for working 160 hours a month. If they aren't doing that, they needs to talk to landlords, healthcare, whatever as they have more pull than you do. If they aren't willing you pay you that amount, let them know you can't afford to work for them.
Now odds are you're being picked up for part time. This means they want to avoid paying full time which adds to your stress and workload since you now have to juggle more jobs, more work and less reliable funding to pay your needs by this rule. Because if this, you can expect less than 160 hours a month even with multiple jobs.
So for part time work charge 3 times the number because of the problems part time work brings up.
This is the invisible union. Businesses work together to pay workers as low as possible, workers can band together to charge a fair price. They don't like it? That's their problem. Businesses are bragging about record profits year after year at everyone's expense.
So arm yourself.