Because you entered illegally, you will most likely need to return to your home country and proceed through consular processing and a waiver.
Immigration attorneys' fees differ, just as their service differs, but I will give you an idea about the USCIS filing fees which are universal.
I-130 - $355
Immigrant Visa Application Processing Fee - $400
Affidavit of Support fee - $70
I-601 (waiver) - $545
Then you must consider travel costs, living apart, etc.
I highly suggest you discuss your matter with an immigration attorney familiar with consular processing and waivers. Be careful of attorneys that charge very little on their waiver process, make sure to ask about their success rate and experience and with which countries. I have experience in this area and have done many successful waivers. Note, as long as an attorney is licensed in one of the United States, they are allowed to practice US Federal immigration law.
The above is general information and does not create an attorney-client relationship.
Law Office of Ruby Lichte Powers
5535 Memorial Drive, Suite F-130
Houston, TX 77007
Tel: (713) 589-2085
Fax: (713) 589-3101