First off if you want to run C02 on a 100 gallon you'll have to go with a pressurized system. This is a large outlay at the front end but not that expensive to keep running. However CO2 is not needed if you choose your plants carefully and balance out the lighting. It is a lot trickier to keep the algae under control though. Dry fert's online are really cheap and a years supply will run you under $30 if you don't use C02 (depending upon your stocking).
My largest cost for my 125 is the fishfood. Buying high quality food really makes a difference to the health and colors of my fish but it takes a bite out of the pocket book.