Can I upgrade my iPhone 8 Plus from 64G to more? How much does it cost?

You cannot upgrade storage on a device itself. You would have to buy a different device.

Note there's different kinds of "storage". Click on this link to "Learn the difference between your device and iCloud storage, ways to manage your space, and where to buy more if you run out." --> https://support.apple.com/HT206504

Take-away:

1. Storage on a device itself is fixed and cannot be upgraded. You have to change your present device for one with more capacity.

2. iCloud storage is on Apple's main computers. It is not a hard drive in the sky but by using optimization settings on your device it can help free up device storage. Lesser used items and larger file versions can be stored in ICloud and you can get access while connected to the Internet. It does not turn a 64 GB storage device into a 64 (or whatever) + 50/200/2TB GB device but if you have a lot of things that aren't being used much, or you don't mind only having lesser quality photos on your device it can free up extra space so you can make the best of the GB you do have on the device itself.