Assuming this is your biological daughter, then she already
automatically became a US citizen under INA 320 the moment she entered the US, as she was a US permanent resident, under 18, who was living in the US in the custody of a US citizen parent (you).
As your daughter is already a US citizen, you can simply apply for a US passport for her at any time, without needing to have obtained a Certificate of Citizenship first. For evidence of her US citizenship for applying for a US passport, you guys would present her birth certificate, her green card or other proof of permanent resident status, proof of your US citizenship, and your marriage certificate as proof of custody. See the US passport
Citizenship Evidence page -> Examples of Secondary Citizenship Evidence -> I was born outside the United States -> I became a U.S. citizen through my parent who naturalized or through the Child Citizenship Act of 2000:
- Your foreign birth certificate listing your parent(s)
- Evidence of your parent’s U.S. citizenship such as a U.S. birth certificate, Consular Report of Birth Abroad (CRBA), or naturalization certificate
- Evidence of your permanent residence status. Examples include:
- Permanent Resident Card/Green Card
- Foreign passport with the original I-551 visa entry stamp
- Your parents' marriage certificate (if your parents were married when you legally entered the U.S. and before your 18th birthday)
You guys can also file N-600 for a Certificate of Citizenship for her. As you already know, it is pretty expensive and isn't really mandatory for anything if she already has a US passport, but some people like having a certificate.
(By the way, how many years you were out of the US has no bearing on whether your daughter
got US citizenship at birth. Rather, it's the cumulative total of the amount of time you were in the US in your whole life (before her birth), that matters. For a child born in wedlock after 1986 to one US citizen parent and one alien parent, the US citizen parent (you) had to have been physically present in the US, any time in your life before her birth, for a cumulative total of 5 years, including a cumulative total of 2 years after you turned 14. It's possible for that to be true even if you have not been in the US for the 20 years before her birth. I am guessing that you failed to meet one of those two conditions.)