We are a couple looking to adopt a cousin's daughter. What's the cheapest way to do this in illinois?

We are a couple aged 23 and 24 years old and my girlfriend already has guardianship of her cousin's 5 year old daughter. We plan to change our last names legally before we start the adoption process. Would it be cheaper to have my girlfriend adopt her because she's legally her relative and already has guardianship and then later on i petition for a second or co-parent adoption or would it be cheaper to just file a joint adoption petition together initially. We're not only looking for the cheapest way but also the best and easiest way to do this. Also just to deter any rude comments we've been raising this little girl since she was 4 months old and she is now 5 years old and completely understands that she has 2 moms and that most kids have a mom and a dad but some are different. We want to adopt her to complete our family and clear any insane legal issues later in life for her with us not being her legal parents. Thank you all in advance for all your help and i'll definately award the points to the best answer! :-)