I doubt BMW ever sell a "base" model. A quick look at the local dealer inventory shows zero base models. Every one on the lot is full of nice expensive options which add at least 10K to the cost. Obviously the dealers make sure to never order cars without a bunch of options.
So the big question is will the Model3 still be on online order and a customer config, or stuck with the current (for everyone else) dealer order method.
All the more reason why Tesla Motors should no it adopt sales through 'independent franchised dealerships' any time soon -- if ever. Technically, it is possible to see a base version of the BMW 320i on the BMW USA website. Rots-o-ruck ordering one from a local dealer for MSRP, plus destination, sales tax, and registration fees alone. Not so with Tesla -- you will actually be able to order a base model, you just need personal discipline to manage the feat. ;-)
Do you mean the Prius is what you consider Mass Market ? ... And BMW sells about 12 K 3 & 4 series per month.
In the United States, the BMW 3-Series outsold the Toyota Prius in 2014. They are both mass market, because each was in the top twenty passenger cars for the year, reaching #16 and #20, respectively. Each moved over 100,000 units, only three cars in the top 25 passenger vehicles missed that mark.
The Tesla Model ≡ will go after BMW 3-Series because it is the sales leader in its market segment, just as the Mercedes-Benz S-Class is the perennial leader in its price range. The 3-Series also happens to be the single best selling BMW product worldwide. So if Tesla Motors can match or surpass 3-Series sales, which Lexus, Acura, Infiniti, Mercedes-Benz, AUDI, et al have been chasing for decades, within three years of the Model ≡ launch, it will make a powerful statement.
Look at it this way: If 500,000 Model ≡ are built in 2020... And 40% of them are sold in the United States... That would be 200,000 units for the domestic market. More than enough to crack the top twenty in mass market sales. I expect Tesla Motors will be rather aggressive and try to reach that level of US sales a year or two earlier than that for Generation III vehicles.