I expect Dual Drive to be free and I expect range to increase.
Point 1: Why decrease the Tesla market size by increasing prices or adding complexity via producing multiple drive-trains?
Tesla had been targeting a gross margin of 25% and last quarter they met it. Their current goal is to grow this to 28% by the end of the year. Their ASP is 92K or so, so we're expecting this to be about 2.75k per car they need to tweak out. Remember that they've invested heavily in new technologies (camera system, dual-drive, etc) that are not counted as gross margin but R&D (100M write-down). That said, rumors re: supplies willing to bend backwards for Tesla today means that their seats, lights, steering wheels, etc have all come down in cost. I'd expect the cost savings to easily cover 10% gross margin.
So why give away dual-drive for free? Well, the Tesla Model S runs from 70-120K today. Tesla is more profitable for each increasing production day. The number of people in the market for 70-120K cars is dwindling as they all buy Teslas Model S & Model X. They need to sell every car they make until 2017 when they launch the Model III. Does it make sense to lower the # of potential buyers by raising prices? How can you maintain a healthy backlog over 2 years? By making your product so desirable, it will guarantee you income until your mainstream vehicles are available. Let me put it this way, if Tesla sells 75K AWD Model S between today and Model III unveiling and charges $5K, they will make an extra $0.375B in sales out of $18.2B, or 2%.
The question is, does Elon believe 2% is worth the trouble of maintaining 2 separate drive train production facilities? Tesla is not built like Toyota where each individual drive-train gets a huge benefit for economies of scale. Creating 2 production drive-trains drains the production efficiency. From a $s perspective, it is going to cost Tesla money.
Point 2: Range is going to go up, without increasing the battery!
So dual-motors will give us 2 VFDs. VFDs have an efficiency rating for their frequency range. I'm trying to find this, but I expect that at their max frequency, they are less efficient. I'm trying to find documentation on this. I have been unsuccessful. =( However, from everything I've learned about variable frequency systems, this has always been true. (Electrical Engineer)
Point 3: You go faster with AWD.
Dual-motors also has dual gearing. This means we can expect to see constant torque available across a wider speed. Currently the top-speed is limited by the gearing. Not much else to add.
Point 4: Elon says so.
Elon had said in Vanity Fair *last night* that the internet got it right, but doesn't really understand the scope. I think he's gotten a huge pickup in efficiency and is ready to unleash this on the world with minimal costs. That would blow people away. =)