Because of the hills and mileage, initial costs are high but I do save about $40 each month.

4 jersey (various weights for differant seasons)
1 rain gear (good set)
1 short
1 tight
1 really good pair of cycling shoes
1 really good bike (used)
====
$1600 including lights etc

$50 /mo gas when weather is bad, drive to different office/church/hauling/night outings
$30 /mo bike maintenance (tubes, replace this or that whatever)

Bike $80/mo vs. Car only: Monthly gas $120