As in the title. The game in question is Vampire: the Masquerade 20th Anniversary edition. For some reason I cannot find it anywhere in the book.
Rituals cost no XP to learn.
Ref page 212, middle of the right column, VTM 20AE corebook:
Additional rituals are learned separately, as part of a story; players need not pay experience points for their characters to learn rituals up to the level equal to their overall rating in Thaumaturgy, though they must find someone to teach the rituals in question.
There is no experience cost.
"You learn the ritual from someone who already knows it or a book/scroll. This can take anywhere from a few nights to months or even years."
Source: VTM 20 ed book. Pg. 230 Top left hand corner start.
Since no xp cost is mentioned, it is able to be assumed that the cost is inherent in the cost of your Thaumaturgy or Necromancy.