It costs about a $ 1000 for someone who has a green card already and meets all the other criteria required if you fill out the paperwork yourself, know how to follow up, etc. This expense adds up if you hire a lawyer and want to make sure all the forms are done correctly. Many are scammed by lawyers, lost thousands of dollars and still do not get their citizenship., and are not reporting it to authorities, and if denied , have to start a new application and pay all the fees again!
" If we add up all the various fees required to come to the US and obtain citizenship, the total falls somewhere between $4,000 and $11,300. Most people end up spending between $6,000 and $8,000. " Source:
http://www.doughroller.net/smart-spending/costs-of-becoming-a-us-citizen/