Of course it costs valve something to offer more versions. It's all inventory that has to be created, tracked, shipped, warehoused. Going from 2 to 4 versions in hardware configurations at that scale isn't cheap, and most certainly not free.

And yes, their stock is also limited. How limited exactly we don't know, as I don't think they've said that publicly. You also have to decide on some number of devices to not equip with memory/storage to sell as barebones, which will have some margin of error that is now sitting around unsold, or everything sells out and they could've sold it just as well with memory/storage and made more money.

But the point isn't to become a vendor for cheap-ish computers. The point is to sell computers so they can sell games, and offering a barebones version is unlikely to justify the considerable overhead that would require.

Finally, they are trying to create a somewhat default/reliable platform for steam games in terms of performance. Game studios are going to make presets for this so the players can expect a certain performance. If you only pop in 4 GB (or they get resold on the used market for with 4 GB) it might perform way worse or even crash. They probably also don't want to inflate or encourage the number of devices that fall into that category.