creatw a private forum in my wordpress site

hello there, i need someone to establish a forum that is secure with a payment gateway inside my wordpress site . my site is already established .. i just need a private members area to where visitors can subscribe the membership to the forum .so if youcan preform this work for me let me know . keep in mind that im on tight budget and will not pay more than a 100 dollars .. forums should be professional looking and everything have to functional before payment released. thanks a lot OMAM