When a function is called the program counter and any parameters to pass to the function are pushed onto the stack. The parameters are then popped off the stack for the function.
When you exit the function the program counter is popped off the stack. The overhead depends on how many parameters you are passing to the function, you can improve this by passing parameters as pointers, wrap up all your parameters into an object or structure and pass a pointer to that.
Using an inline definately has it's uses, it effectively turns your function into a macro and removes all the stack operations, however if the function is called from many different places I wouldn't use inline, just try to optimize the function and its parameters.