I'm not sure I'm completely following your math (I probably just didn't catch a step, though).
However, I do think you're missing a key differentiator between Tesla and BMW. If you want to back into costs, you can't look at BMW's MSRP to compare it to Tesla. MSRP has dealer profit+expenses built into it and it's difficult to know what manufacturer margin looks like compounded with dealers' margin+expenses. Tesla obviously also has "dealer" expenses, but that hides in SG&A.
Instead, I think you should start with BMW dealer cost and then apply BMW automotive margin.
Going backward on BMW:
1. MSRP: $34,000 (including destination, according to Edmunds)
2. Dealer Cost: ~$30,000 (That's based on Edmund's true market value pricing of $31.3 - dealer has to get something).
3. BMW Margin: 15% (that's admittedly average-ish, and could be lower on a 320i).
4. BMW Cost: 25,500
5. BMW Drivetrain: 7,500 (your assumption)
6. BMW Everything else: 18,000
Now the same exercise for Tesla:
1. M3 Price: 35,000
2. "Dealer" Cost: 35,000
3. Tesla automotive margin: 25% (or 8,750)
4. Tesla cost: 26,250
5. Tesla drivetrain (my bet is 50-55 kWh):
- 8,000 for 50 kWh ($100/kWh)
- 10,500 for 50kWh ($150/kWh)
- 10,000 for 70kWh ($100/kWh)
- 13,500 for 70kWh ($150/kWh)
6. Tesla everything else: 12,750 - 18,250.
I happen to disagree on 2017 battery cost (I think they are 150-ish/kWh today before GF, but that's another conversation) and think they will be $100-ish by 2017.
So, given my example above and $100,kWh, if base M3 uses a 50 kWh pack, Tesla has $250 more to produce the same car as BMW. Tesla probably does have somewhat higher supplier costs, but Tesla might be more vertically integrated. More importantly, labor costs are likely cheaper in Northern California than Germany.
And, most importantly to this thread, my back-of-the-napkin math still leaves room for 25% automotive margin at $35,000 sales price -- not selling at cost.
If Tesla wants to sacrifice some margin (remember, it has $8,750 to work with), it should even be able to produce a higher content* car than a 320i for $35,000.
*Remember, it is very likely Tesla can already produce a higher content car than a 320i with some features that don't cost it much of anything. A lot of Tesla's higher content is simply software (with low marginal cost) -- center screen, navigation, maps, internet connectivity will almost certainly be included with a base model M3. The equivalent "technology" package for a 320i has a $2,500 invoice cost.