I like to track the cost of my consumables (by which I mean mostly tires, chains and brake pads, with lubes, grease, etc. as insignificant items in this budget) to about the cost of gas for my car on a per mile basis. Yes, I know this disregards the other car-related costs like maintenance and insurance, but since I keep the car anyway and its disuse has a maintenance cost too, I figure this is a nice target. I also find it gives me a pretty generous budget for tires.
I do keep track of the per mileage cost of my bikes, but I've never tried to work them in. My most expensive bike (2008 Kona Jake, $900) is at about 11 cents per mile right now, not counting parts I've upgraded. Counting parts and extra bikes, I'm closer to 45 cents per mile over the past four years.
If I were in your shoes, I wouldn't bother with a $450 bike. Treat yourself to something nice.
Last edited by Andy_K; 02-28-12 at 12:01 PM.