Do you even have kids? Do you know how much money it costs to raise two kids? $900 for half of the expenses is not greed.
2 extra rooms for housing: $500-1000 (the higher amount is sometimes needed to move into a better school district. if you are single or married without kids, you can rent a small apartment without regard to the local schools)
food for two kids: $300-400
Utilities: $150
Cell phones: $100
School supplies: $ 100
Extracurricular activities (limit one per kid): $200
Medical care: $100-300
Clothing: $150-300
Personal care (toiletries, haircuts, etc): $75
Extra furniture, bedding, towels, dishes, etc, household items needed for extra humans in the house: $50
Toys (anything from dolls, legos, books to x-box, skateboard, iPod): $50-150
Gas to drive them everywhere: $40
That's just the simple and conservative list off the top of my head. About the only thing not necessary is cell phones and toys, but I have provided both to my child. The cell phone mostly because I am a paranoid mom that likes to be able to reach my child. The toys because I actually get a 1/2 -1 hour of peace if my child has a book to read, a game to play in the evening.
The lower amounts total to $1825, which is in the ball park of the $900 a month child support. And that's no emergencies, no special needs, no chronic conditions, no winter clothing (grew up in MN, when I moved back I spent about $600 on winter clothes for my kid and myself), no athletic or musical talent that strongly encourages more lessons, no college prep, no braces or glasses, no driver's education, no prom. It certainly does not cover court costs to handle custody and support issues. It does not cover loss wages sitting up with first one and then the other because they can never get sick at the same time, moving fees because their school district does not have Latin or AP Calculus or a dance team, night classes so you can get the better paying job and hopefully pay for their college.
When you divorce, you still have kids. You need to realize that kids still need food, shelter, etc. If you receive a summons for child support, the responsible thing to do would be to start setting aside money to pay the support. One can hope that the ex will be understanding during hard economic times, but remember, you did divorce them for a reason.
Don't ignore the served papers, you need to respond. She can ask for more money, it does not mean she will get it. She can ask for her lawyer to be paid for, it does not mean she will get it.
Do your research, document what you can and be prepared to bring the documentation to court. If at all possible, get a lawyer.