I don't think bike commuting is saving me money

I commute on a Cannondale 500 Mtn. bike. I've added fenders, and lights. It's very geeky now. My last purchase was a set Ritchey Tom Slick MTB Tires. They're 1" slicks and a ton faster than my knobby Mtn. tires.
But more is never enough. I started thinking "Wouldn't it be nice to have a second set of wheels so I can swap mtn, and street tires painlessly?"
So I found a set of Mavic 517 rims with XT hubs (just like the ones on my bike) on EBAY.
But that's not enough. Oh no.
After consulting endless online gear calculators, and the amazing Sheldon Brown bicycle site I figured I knew how to kick things up a notch.
On EBAY I found (and purchased) a 12-23 cassette with a much "tighter" range than the 11-30 that's on the bike now. My hope is that this will make the perfect no-hills-involved commuting machine! Of course I had to order the installation tools as well, so even with the EBAY used part purchases the project is costing me money.
Pictures and installation reports to follow.