My XH makes a butt load of money and I don't get $2k/month for one kid. If I were you, I'd tread very lightly.
I know you're a couple years away, but my D17 turns 18 in March, and life as we know it will change greatly once she is an adult. She will qualify for SSI (covers shelter & food) and the support formula can and will change for her once my D17 turns 19.
If I were you, I'd make sure to consult with an estate attorney who specialized in elder and disabled law (perhaps the one who set up your special needs trust) and make sure that you don't have to revisit this anyway in a couple years. Child support cannot be written for life. So please get this done the right way so you and your H have clarification on how this will affect all of you.
For the record, federal SSI is about $720/mo and has a formula to go down depending on what the financial situation is for your D15 at emancipation. You have to charge her rent in order to keep it. I can't even begin to touch on this because it's very unique, so please, PLEASE make sure that your agency or community board is brought into the equation too. You're probably okay for now, but things are very different once they turn 18.
You truly need some specialized legal advice here...
Good luck!
Betsey
"There are only 2 ways to live your life. One is as though nothing is a miracle. The other is as though everything is a miracle."