Its $20 to rent a range for an hour and $10 to rent a handgun, but roughly how much would the ammo we would need cost?

Your total cost will be around $100. Your charges will depend on the caliber weapon you rent.

The bigger the ammo the bigger the cost .. Then how many rounds do you want to put downrange? Call and ask

What would it cost on average for a small caliber gun?

You need to call to find out different calibers different prices

Depends on the caliber of gun you rent and type of ammo you choose.