I'm not advising these particular rims (wrong size), but they were a junkyard find, at $20 each. The cost savings over new rims easily outweighs the fuel savings.
For the Civic in my signature, I went around with one of these and weighed various options in my favorite junkyard. The rims that looked the lightest were actually not even close.
I too advise against upsizing. Even if the weight is the same, more of it is toward the outside, where it will have greater inertia. The close the rotating mass is to center, the better.