The wheels are moderately expensive if you have to pay list price. A note, though, the i3's wheels are reported to be forged verses cast, which means that they are stronger than more typical alloy wheels. When you opt for a larger diameter wheel on any car from any brand, your risk of wheel or tire damage increases. While not on this car, my worst experience was two flats the same day (nails/screws, not impact damage)...haven't had any tire issues on a car in years...some of it is just plain luck, and some of it is close attention to the roadway whenever possible. If you don't follow too closely, you have more opportunity to avoid road issues, something way too many drivers in the USA do not follow.
FWIW, from my living and visiting Germany over the years, I find that their roads are generally VERY well maintained, and a pothole is a very rare thing. That may be one reason why, on their other models, they often specify RFT...those tend to be a little stiffer than an equivalent 'normal' tire, and there is not as much of a compromise when the roads are smooth. And, other than the drag and efficiency issues on the i3, they chose the wheel/tire sizes they did for overall efficiency. So far, I've not had an issue with the i3's wheels or tires...not to say it couldn't happen tomorrow.
Given the nature and capabilities of the i3, I personally see little reason to opt for the 20" wheels.