Yes the cost is very expensive in America. Rather you can fly down to the India and get the IVF done and I bet after all expenditure you will still save the money. As you have done your Tubal Ligation the natural path of Ovum is now blocked and hence it is not possible for your ovum to meet your husbands sperms and make you pregnant. In IVF your Ovums are extracted through OPD based surgical procedure and then made fertile with the help of your husbands sperms in a petridish this is now called a embryo and this embryo is transferred in your uterus and the pregnancy proceeds as usual... You are already having two kids.. At Advance age of pregnancy the chances trisomy increases that may make you child suffer from at least Mental retardation.. Talk more with your genetic counselor.. Rest the decision is your own. I hope the information provided is enough sufficient for you...