When networks of self driving cars start appearing, it'll reduce the cost so much that it'll trickle down to the 10%, then 50%, then to the 100% - much like the original car or the fridge did. When I used to work in downtown SF, taking an Uber from my house to work was only 2x as expensive as the bus, even though it was more like 4x-6x a few years ago. Over time and as technology/infrastructure improves this will go down to 1x, and then even below.
It'll be a great future where people don't own cars anymore (except for a select few who will for recreation, much like people still ride horses for fun now), and instead pay a monthly subscription fee for X miles of driving around per month. Much like cellphones, there'll probably be things like family plans and so on. Your kids will get a self driving car to pick them up from school, which will promptly drop them off at home/sports practice/etc. The car will then just drive to its next request, thus completely removing the need for parking (which will have a wonderful impact on American urban areas- larger sidewalks, no more need for giant parking lots everywhere), heavily minimizing traffic (no stop and go when all cars on the road can sync up in real time), heavily reducing lethal road accidents, etc.
Self driving vehicles will do for transport what the fridge did for food or the internet did for communications. It's going to be very, very exciting and I wish more people realized it so we could make it happen as fast as possible (e.g. a really forward thinking city could easily make a subpart of its downtown area open to self driving vehicles exclusively by 2020).