This is the official website for the cathedral: Catedral de Granada. Prices are listed (5 euro individual, 3,50 students, kids under 12 and disabled free, admission includes audioguide).
There is no link to purchase individual tickets, just group reservations.
We went last April, mid-week, late morning. No line, no crowd. It is a beautiful cathedral -- very different feel from others in Spain (although, like so many, still built on top of a razed mosque).
Enjoy your visit!
Edited to add: looks like if you are there on a Sunday afternoon, there is free entry that must be reserved ahead of time. There is a link at the bottom of the "horarios (hours)" section on the Cathedral web page. I don't know anything about this visit, whether it's the same as a general admission on other days, or limited admission because of Sunday, but thought I'd post the info and link in case you are interested: https://entradasgratuitas.diocesisgranada.es/view/event/catedral-en-catedral/