Oh, how are you going to live in a place for $500, if he only makes $400 and you're not working.
First, you need to sit down and write out a budget for the following:
Rent (moving into a place ususally requires first, last and security, so 3 months rent upfront)
Utilities, like electric and heat
Your cell phones
Food for 2 people
Transportation....cars which may include car payments, would include gas, insurance, maintenance. Or public transportation.
Do you expect to have cable TV?
Clothes, entertainment, birth control....I'd say that's a must right now....Birth Control!
I don't know that your moving in together is going to help your relationship at this point, considering the stress you'll have of not having enough money to survive.
However, this is an opportunity for both of you to figure out realistically what it would take to live on your own.