What does average price paid mean?

It means the average price. If you take all the prices that a car sold for and average them you get the average price. Dealer invoice is what the dealer paid for the car.

Where are you getting the price all vehicles are selling for? I'm looking at BMW i3's and the number seems to low from what I'm seeing in the market.

You can check prices on several sites such as Car Gurus, Edmunds, Blue Book etc. Remember that car dealers mark the price up so you have nothing to lose by offering them less.

A dealer invoice doesn't mean a thing. It's a paper that car dealers use as a con to make you THINK that's what they paid for it, when in truth, it's the MSRP. Dealers don't pay for cars; they're sold on consignment, and the maker tells them how much to list it for.