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: 
There are a few issues here.

* If you have a smart enough compiler, it will do some automatic inlining for you even if you did not specify inline. On the other hand, there are many things that cannot be inlined.
* If the function is virtual, then of course you are going to pay the price that it cannot be inlined because the target is determined at runtime. Conversely, in Java, you might be paying this price unless you indicate that the method is final.
* Depending on how your code is organized in memory, you may be paying a cost in cache misses and even page misses as the code is located elsewhere. That can end up having a huge impact in some applications.
For most functions, their is no additional overhead for calling them in C++ vs C (unless you count that the "this" pointer as an unnecessary argument to every function.. You have to pass state to a function somehow tho)...

For virtual functions, their is an additional level of indirection (equivalent to a calling a function through a pointer in C)... But really, on today's hardware this is trivial.