Monthly Payments: 

If you purchase, lease or rent the vehicles there is an associated monthly payment that goes along with that.  You must ensure that your budget can accomodate the monthly payments for your combined fleet.

Insurance: 

Auto insurance is a must in the U.S.  The minimum level of coverage that is required varies by State. 

Get the maximum amount of coverage and make sure that you get the "Symbol 1 (Any Auto)" coverage.  Business auto policies are written with symbols, ranging from 1-9.  Symbol 1 provides the maximum protection and Symbol 9 provides the least amount.  Symbol 1(Any Auto) allows you to get immediate coverage for any new vehicle that enters your fleet, so that all of your vehicles are always covered. 

Consider getting "per-occurence" catastrophic coverage versus "per-vehicle" coverage.  This will limit the total amount your company will have to pay when a single event damages several vehicles at once.

Gas:

You have to pay for the fuel used in the vehicles.  The more driving you do the more it will cost.  If you buy fuel in an area where the cost of fuel is usually higher than the national average your monthly cost will be higher.

Maintenance:
The vehicles has to be maintained.  The oil must be changed regularly and other fluids must be replenished.  Brakes get worn out after a period of use and must bed replaced.  Damages to the vehicle must be repaired.  There is a cost associated with this.

Registration Fees:
All vehicles must be registered in the stated where they reside.  Registration fees varies by state.  It also depends on the type of vehicle and the age of the vehicle.  New vehicles cost more to register than used vehicles on average.  Vehicle emission inspections is also a requirement to get your vehicle registered in most states.  New vehicles are usually exempt from this for the first few years.

Fleet Manager:
If you have a fleet you need a fleet manager to manage the fleet.  Depending on how many vehicles you have this may be a full-time position or a part-time position.

Fleet Management Software:
The Fleet Manager needs a fleet management software to manage the fleet.  The fleet management software could be a one-time cost or a monthly cost depending on how you purchase it.

Early Termination Fee:
If you have Lease Agreement you will have to pay a termination fee if you terminate the agreement prematurely.

Parking: 
If you have a fleet you have to have a place to park it.  Depending on where your business is located this could be an expensive proposition.

State Taxes: 
Some states have tax requirements for leasing vehicles.