Hi Jason -
Congrats on the new car!
As you've already seen by the responses here, "most cost effective" is a very relative term. It all depends on how much you drive, when you drive, when you want to charge, how fast you need that charge to be, which battery you have, etc...
In general, home charging of a Model 3 goes from 3 miles-per-hour, all the way up to 44 miles-per-hour. How fast do you need your charging to be? If you don't drive very far, on a typical day, then slower charging may be all you need. If you have "off peak" electricity rates you want to take advantage of, then perhaps faster charging in the middle of the night is what you need. If you want the best of all worlds, then you want to be able to charge as fast as possible.
You'll find all kinds of helpful advice here. But everyone will have a different opinion on what is best. One person will tell you to just use the included connector (its not a charger) because that's the cheapest route. Another person will tell you should at least get a 240v outlet so you can charge faster. Another person will say you're already paying an electrician to put in that outlet, might as well spend a few more $$ and get the wall connector (and a potential 30% back on taxes).
Electrician rates vary wildly, as does the work needed for your specific dwelling's layout.
Once you figure out how fast you want to be able to charge, then you can decide what to get. The included connector will charge your car at a maximum of 29-30 MPH if plugged into the appropriate outlet (240v, 40Amp). A wall connector will charge at a maximum of 44 MPH if you have long range battery (max of 30MPH for standard range battery).
