dont forget to budget for the other essentials for riding in traffic like:

helmet
taillights
frame pump
tool bag (enough atleast to change flat tire)

as far as im concerned, all cyclist should be required by law to use the first two items at all times (when riding on the streets). The next two just make good sense to me.

of course there are other items you will probably want eventually like:

water bottles/cages
bicycle specific clothes, ie padded shorts, gloves, rain jacket....
maybe a rear rack or backpack
clipless pedal system