I wouldn't overline your rod. Stick with the line weight that the pole is designed for. If you overline to start off, you're not properly learning how to fish the rod. Only experienced fishermen should ever try overlining since they're already experienced casters. Just my opinion!
You could try your local fly shop for a demo line that's been used for practice casting and you could save a bundle, but you'd still be getting a premium line. The Concept fly lines are inexpensive and cast well. I have one that my wife learned on, and it's a great line for the price.