No
You can not buy a 135 certificate, you can obtain one, through all the certification, inspection and proving runs required by the FAA. That can add up to well over $50,000 depending on equipment.
You can buyout a 135 operator and assume their certificate, maybe that is what you are referring to.
You can attach yourself to a 135 operator (get on their certificate) with a formal agreement. You will be required to bring your MX Dept, Training to 135 standards, possibly additional insurance. You officially become part of that operator's certificate, so expect other requirements such as cleanliness, uniforms, customer service, etc.
As far as monthly fee's. Forget about it. Arange a dollar per hour agreement with the Operator. Monthly fee's are crazy unless it is for additional insurance and maybe service fees such as records, dispatch, etc.
I have heard Averitt Air in Nashville is good about doing what you want. They have reorganized a bit this last year, but I hear they are solid and growing steadily in the Charter industry.
Good luck