The 2017 27" iMac still has four user-accessible memory slots that take 2400 MHz DDR4 memory.

You can easily find the correct memory here on Amazon at a fraction of the cost Apple charges you.

Progressive upgrades (processor, bigger HDD or an SSD, more memory at purchase, etc) are extremely expensive and add a disproportionate amount of cost to the iMac

It's the first iMac in a while that has anything resembling upgradable memory.