A new model of the Surface Go is now being sold on Microsoft's website, which sits right in the middle of the two previously existing variants. Previously, you could get 4GB RAM and 64GB storage for $399 or 8GB RAM and 128GB storage for $549, but now there's an option for 4GB RAM and 128GB storage for $499.
You might recall that this model did previously exist, but Microsoft wasn't selling it. It was an exclusive for Costco, and it came bundled with the Type Cover for $549. The one that Microsoft is selling does not come bundled with the keyboard, so that's still an additional $99 expense.
More importantly though, it lowers the barrier of entry to the 128GB SSD. The lower end $399 not only has half of the storage, but it's eMMC storage, which is much slower than the SSD models. This could be a cost-effective balance between higher performance storage, and 4GB RAM.
The new Surface Go variant is only available in the consumer SKU, which ships with Windows 10 Home in S mode. Surface Go for Business, which ships with Windows 10 Pro, still only has the two original models.
If none of the three models fit your needs, don't worry, as there are still more to come. The original spec sheet listed a model with 256GB SSD, which I'm told is still coming. It's likely the promised 4G LTE model, which is slated to arrive later this year.
If you want to check out the Surface Go on the Microsoft Store, you can find it here.