Is "I am too humble to pray for you" a correct saying?

The first part of the saying is correct, for one should view oneself with imperfection to avoid arrogance and pride, and to increase self-purification and humility. There is no harm in asking for prayers from others or responding in a manner similar to what was mentioned in the question. As for the other part, it is not absolutely correct. It must be interpreted in a sound sense, such as the Muslim's dignity and courage in enjoining good and forbidding evil, and striving in the way of Allah, and not displaying grandeur and abandoning humility. Humility is always required, but not in a way that leads to abasement or debasement.