Question: A lot of literature talks about using inline functions to "avoid the overhead of a function call". However I haven't seen quantifiable data. What is the actual overhead of a function call i.e. what sort of performance increase do we achieve by inlining functions?

Answer: 
The amount of overhead will depend on the compiler, CPU, etc. The percentage overhead will depend on the code you're inlining. The only way to know is to take *your code* and profile it both ways - that's why there's no definitive answer.
Each new function requires a new local stack to be created. But the overhead of this would only be noticeable if you are calling a function on every iteration of a loop over a very large number of iterations.