The Eon Series (Legacy, Eon, Eternity) by Greg Bear is $2.99 on Amazon:
https://www.amazon.com/dp/B071YMZ3RT
and Barnes and Noble:
https://www.barnesandnoble.com/w/the...=9781504045452
Annoyingly, this sale price is not consistent. It's still list price at Kobo and Google Play.
It is available from ebooks.com for the slightly higher $3.99, but still considerably cheaper then list price:
https://www.ebooks.com/en-us/book/95...ies/greg-bear/