New or preowned? I think it does come down to the subsidy and how it affects you. If your household pays federal income taxes (remember that does not include FICA taxes collected to pay for future Medicare and Soc.Security retirement benefits), then keep in mind that the i-MiEV will "cancel out" your federal income taxes, up to $7500 - so the higher your taxes (up to $7500 anyway), the higher the subsidy. You also need to remember that this subsidy is not available to you at purchase time - you'll have to pay or finance the full price of the car, then get the tax credit when you next file your income taxes. Right now (it being June going on July), that means you're half a year away from getting that money back.
Once you know what a new i-MiEV is going to cost you (i.e., sticker minus either your federal tax bill or $7500, whichever is less), you need to compare that with the pricing for used i-MiEVs in your market. A complicating factor is that the 2012 and 2014 models were equipped very differently, so nothing is exactly comparable. In 2012, Mitsubishi offered the i-MiEV in what amounted to three trim levels - ES, SE, and SE Premium (technically, the Premium was just an option package on the SE, but it cost as much again as the ES->SE trim level upgrade), hoping that most buyers would opt for the higher-margin SEs. To add real confusion to the mix, Mitsubishi made CHAdeMO quick-charging standard on the SE Premium, a $700 option on the ES, and not available at all on the SE.
Things are much simpler in 2014. There is only one trim level, ES, and no factory options whatever - everything's standard equipment. 2014's ES effectively combines the 2012's ES "black on black" interior color scheme with the 2012 SE's additional equipment (upgraded radio, fog lights, aluminum wheels, etc.), standard CHAdeMO charging, and a few minor but desirable new features (12A Level 1 charging, heated passenger seat, illuminated charge port). The advanced entertainment and nav system components of the SE Premium are no longer available.
I mention all this because you'll need to decide what features have what value to you in order to make a value judgment. The lack of brown and silver color accents aside, the 2014 is basically the same as a 2012 SE plus CHAdeMO. If you care about quick charging, a 2012 SE (and many ES as well) won't have CHAdeMO. If you're interested in the SE Premium's fancy electronics package, you can't get that for any price in a 2014 (though of course you could install your own afermarket radio). And of course, one big advantage that pre-owned i-MiEVs have is that their prices should reflect the fact that they're competing with new cars carrying $7500 tax credits, a credit their original buyers have already collected (though remember that the cars they bought also had a roughly $7500 higher MSRP comparably equipped). If your household expects to have little or no federal tax liability for 2014, a good condition used SE Premium is probably worth at least as much as the subsidized price of a new 2014 ES, since you can't get the subsidy.
Sorting through all this should allow you to figure out what any particular 2012 is worth to you. That's going to be very important to know, because this is an extremely thin market with very few recorded transactions, so prices in well-known references like Kelly Blue Book will be highly speculative, and I'm not sure there's any such thing as a "fair" price. You can count on dealers in particular to be listing any used i-MiEVs they might have with the highest price they could possibly imagine anyone paying (since nobody really knows what they're worth and the dealer can only negotiate down from the sticker, never up), so if buying from a dealer you'll need to negotiate hard, and that can only work if you have a pretty clear sense of the car's value to you.
