I think the issue is that you’re trying to assign a value to prayer, to quantify what it gets. We do that with indulgences symbolically, but it doesn’t really work with prayer.
When you consecrate yourself to Mary, you are accepting that, as the closest creature to God she knows how best to utilize your prayer to achieve God’s will, which is what all our prayers should ultimately be about.
If this bothers you, don’t consecrate yourself to Mary. Though, I don’t think it should, as she’s going to get the most “good” out of your prayers. She knows better than you because she is already with God. So no, the prayer might not achieve the end you want, but then, we’re never guarnateed that it will anyways. We’re only guaranteed that God will put our prayer to the most possible good.
I do understand your hesitation though. I’ve been contemplating consecrating all of my merits for the holy souls in Purgatory, rather than for my own benefit. (Incidentally, this is also done through Mary, I believe). Basically, I would be forfeiting the remission of punishment due for my sins I would gain through good works and indulgences, and it would all get distributed out to the souls already in Purgatory in need of prayer. This is frightening because as I am now I expect a rather lengthy stay in Purgatory, and that is far from ideal, but I also have to trust God that He wouldn’t accept this sacrifice without recompense… It is frightening to think of losing “control” of my merits (even if that control is only in my head to begin with), but I also know that God will do more good with it than I could. The same is true for your prayers. At the end of the day, whether it’s straight to God or through one of the saints, how efficacious our prayers are is ultimately in God’s hands.