they cost egg currency you have to solve riddles and find or input a code etc to earn, which you then spend on the shop to purchase the egg collectibles
but there's never enough currency to be able to purchase one of every tradeable egg, even if you manage to solve all the clues. so the rest need to be bought/traded for secondhand
and then if you solve every clue first, you can purchase the golden egg. which that requires you to spend all of your egg currency, which means you then have to get all the tradable eggs secondhand
also, ever since the 2016 event, all eggs aside from the annual golden have always been new. so I'm not banking on the staff giving me a break with there being like only 3 new eggs and 2 rereleased ones, or something, much as I'd love them to