You will have to first certify the certificates at Ministry of Education. If it's a WAEC or NECO cert then u will need a scratch card at Min of Education for confirmation with an on line printed copy of the result. I am not sure of the amount but around 300naira per document, that is at Min of Education.
Then u will then proceed to foreign affairs too. You will pay some amount there as well.
If you are legalising a degree (university or other) cert, then I ll advise u to get it certified at your school first, before going to Abuja. Be sure u will spend about 600naira all together per documents!
All the best!