There's another way to look at some mods, say for eg. Headers and duals. You could end up spend big money doing this mod from a purely mpg perspective might not be worth it, but if say your looking for some performance, could be thought of a performance mod that one day may pay for itself, same with OD. OD is more of a performance mod because it allows you run deeper gears.
But if it is to purely to save money be careful how much you spend.