Murphy's law = Anything that can go wrong will go wrong, so in your case you will need to pay the higher of 2 costs. By the way, your quote clearly gives you answer, does not it?
父母的I-485已经在07大潮中递交。小孩当时在国内，签证问题没有来得及来美国。下
个月current.
8岁小孩交I485的费用是多少？$635 or $985?
有一点疑惑，因为I-485 instruction 写"The fee is $985 for applicants under 14
years of age who submit Form I-485 independent from other family members.
The fee for a child under 14 years of age will be $635 when submitted with
the application of a parent under section 201(b)(A)(i), 203(a)(2)(A) and 203
(d) of the INA."
万分感谢！！！
