Doubling the size of everything, and with it the prices is a fairly poor way of making a pricing assumption.
The vehicle is not double the mass, length, frontal area and therefore also doesn't require double the drivetrain (batteries/motors etc) to propel it. Twice the functionality is also a vague metric for comparison, as functionality is a consequence of design, not just material use and doesn't necessarily impact fabrication cost. A considered calculation will show that the prices are close for the lower spec ones, but not impossible, especially considering that we don't know what supply contract costs they have to compare things too.
As a material SS can be cheaper that aluminium, and is definitely so if you leave out the stamping machines, fabrication and paint shop that aluminium requires and SS doesn't. I'd expect the actual body of the CT to work out cheaper than a M3 despite the size increase of the body. As for drivetrain, the frontal area of the CT is not significantly larger 15% (4sqft) than a MX and although the drag co-efficient will be higher, overall the battery size most definitely does not have to be doubled to achieve those ranges. More like 25-35% more battery capacity should do.
The other factor that needs to be assessed in all of this is that because they are using existing production lines from the Model 3 manufacturing, a lot of the development costs can be reduced for the CT. For M3 they also rolled out there 21700 cell production line, new drive train components etc plus new aluminium fabrication, that all needed to be developed from scratch. The reuse of the drivetrain fabrication means they don't need to be recouped through the CT price, and can essentially be made at maintenance and material costs instead. That's the beauty of skateboard design and high performance EV motors. They're using the M3 drivetrain in the semi as well for the same reason.
The point is that it is possible, if you use the right data input for your assumptions.