If it's already stocked prior to the month beginning would not take much to restock what little you would use, but again would depend on the type of calls and how aggressive the protocols are. Don't forget fuel costs, insurance, etc.
Now if you are also wanting to know what it will cost to staff it on top of stocking it you will have to factor in benefits, taxes, utilities, etc. It will be about $400,000 per year for a BLS ambulance. If you go to a Paramedic level ambulance it jumps to about $600,000 per year to staff, stock, and house an ambulance. These figures presume you already own the ambulance and the station. If you have ambulance payments and a mortgage or rent it goes up.