Let's make it simple.
Unless the magus create it himself as a lab activity, a longevity ritual designed before the start of the saga costs 1 pawns per magnitude up to the fifth magnitude and 2 pawns per magnitudes 6 to 10, plus the cost of activating the ritual (based on age).
No longevity ritual higher than level 50 during pre-play advancement.
So a level 40 longevity ritual would cost 11 pawns to acquire. If the magus is 36 years old when it is activated, this costs an additional 8 pawns.
The cost of a longevity ritual is not reduced by the Mercurian Magic virtue.
Edit: I've just added it to this post: About the Ars Magica 5th Edition: The Heir of Laimunt Valley category - #3 by Arthur