Actually resale values (technically not residual values since the latter is contractual and may or may not reflect expected resale value) depend on many variables other than the vehicle. In practice automotive leases have two components; money factor-translates to interest rate and residual value- the contractually set vehicle value at end of the full term lease. Either or both can be 'subvened' in which one party subsidizes either interest rate or residual value or both. Tesla subvened residual value for both leases and loans when Model S and X were launched, and the contingent liability for those, while diminishing rapidly, still is on Tesla's books.
New model introduction is a 'black art' for residual value, but entities like Automotive Lease Guide devote major effort to establishing justifiable comparable data, so not much major error takes place any more. They, and others, were very reluctant to bet on Electric vehicles, especially after the gigantic bath that had already been in their minds from the original BMW 7 series (losses of >$14,000 per vehicle), plus a plethora of losses on specific models that seems easily predictable. That all ended out with pretty much zero enthusiasm for Model S, itself in a category that typically has >60% of volume in leases in the US and UK. Thus, Elon personally placed residual value guarantees at effectively MB S-class typical levels. As it happened, Tesla has ended out with negligible losses on that arrangement.
Now to Model 3 where among the most frequent trades are vehicles that range from ~50% leases to ~80% leases in US/UK. The typical segment resale values are very well documented, but Model 3 remains an outlier not only because of Tesla but due to sedan and high performance versions. My personal point of view is that Model 3 leasing will begin in the US Quarter 1 of 2019, probably in partnership with a major operator that can also assume S and X, since the existing servicer is pretty much illiquid right now. Some, like Chase, Wells Fargo and Ally, are past masters of devising creative lease offerings and can fund Tesla operations quite easily. Chase also operates 'captives' for JLR (distinctly branded Jaguar and Land Rover), Mazda, Subaru, Maserati and others. Chase has pioneered some quite creative exotic car deals through the years. Tesla now has the volume and credibility to attract major attention in this sector and can also do some stellar financial engineering for other items (Superchargers?, Stores?, and the list goes on...
The markets will quickly get excited about such deals because they'll reduce costs, facilitate increased sale volume and dramatically reduce direct working capital requirements. Pretty rapid value can be released this way.
