adoption info

HI my name is Ashley i am looking for some info on adoption, i have my 3 sisters kids for last 10- 11 years and wanted to know how much adoption was and also i came from a adopted family as well the family who had adopted us had subsidy, so i also wanted to know if i could get that for my 2 nieces? or is the subsidy only for kids who were in a foster care saystem? my nephew has ssi, so from what i have seen so far he would not get subsidy thank for any info Ashley