Lenovo, like Apple, charges a ridiculous markup on SSDs. However, unlike with a Macbook Pro their laptops are user serviceable.
If you want a 1TB SSD and know how to use a screwdriver, you can save $250 by buying the SSD yourself. If you're willing to start with a 256GB SSD and hold off on upgrading until you actually outgrow your storage space, you could save even more.
The question is for how long will the serviceability stay this way. Lenovo is switching to one soldered RAM slot and o in the new ThinkPad generation. [1]
I own a ThinkPad X1 Yoga (2016) and except the battery life I'm extremely happy with it. Maybe I will switch to a MacBook in the future but the keyboard is a turn off (No F-keys and no Home, End, Insert and Delete keys). On the other hand I like that Apple has only Thunderbolt 3 connectors and I like macOS.
If you want a 1TB SSD and know how to use a screwdriver, you can save $250 by buying the SSD yourself. If you're willing to start with a 256GB SSD and hold off on upgrading until you actually outgrow your storage space, you could save even more.