Child support payments?

can anybody please tell me where i stand with this issue.my american wife moved back to the states,from UK,with our 2 kids 8 months ago,she went under the false claim she would work on my green card.One of our girls was born in the states and one in England.now she wants a divorce which leaves me stranded rom seeing my kids on a regular basis,i miss them dearly!she wants $250 per week child supports plus a contribution towards health care of $300 per month.Is this reasonable?seems very high,i have been paying $145 per week as i only earn approx $520 per week,she earns $300 a week.I have bills left from our marraige and rents are high in London,she has also only offered 2 weeks a year for me to fly over and see my kids in the custody,and will not be affordable if i have to pay $250 per week,please can i have advice on this and thank you for reading this long question.