To give some context, a new automotive assembly plant could cost around a billion[1]; or an old plant could be modernized with a billion[2] -- It is well within the realm of a normal capital expenditure of an auto company.
Industrial Automation is more expensive than you may imagine. A robot arm may be 100k to 200k or even more (hard to cite as prices are never listed on the internet); you also have to have a controller (PLC) and all of the associated sensors and connections.
We already have heavy duty robots capable of high precision positioning; the area for expansion / improvement is in making these cheaper, and in "Internet of Things" style automation.
Modern factories are heavily automated in the body welding processes and paint application.
The only area of the plant unconquered by robotics is General Assembly, where all the fiddly bits are put on - wire harness, carpet pad, carpet, etc. Robots are famously bad at handling bendy things. [3]
1. http://www.wsj.com/articles/why-auto-makers-are-building-new...
2. http://www.wfaa.com/story/news/local/tarrant-county/2015/07/...
3. http://www.npr.org/sections/money/2015/05/19/407736307/robot...
Disclaimer - I used to work at Arlington Assembly, and still work for GM. Any opinions are my own only.