I decided to do some quick research regarding sales for series and one-shots that were focused on Jean. Here are some numbers:
It seems Jean Grey debuted at 71,589, Phoenix Resurrection: The Return of Jean Grey at 145,057, Generations: Phoenix & Jean Grey at 59,452, and X-Men: Red at 97,081.
From what I can gather, none of the series, save for annuals and graphic novel formats, dipped under 30,000 units. They were financially successful.
https://www.comichron.com/monthlycomicssales/2017.html
https://www.comichron.com/monthlycomicssales/2018.html