The cost to operate/man/ventilate/properly clean and maintain a public indoor range pretty much eliminates "free" unless you have some sort of membership that you paid big money for... but then, that wouldn't be free either.
One of the not so local to me, but popular indoor ranges in one of the more "high end" parts of the region charges as follows:
Lane Use Fees
Sunday – Friday
First person: $15.00
Each additional person: $10.00 (up to 3 max)
First person: $20.00
Each additional person: $15.00 (up to 3 max)
And this for rentals:
Our rental policy states that to rent a firearm, you must either bring your own firearm, or have another person with you at time of rental. Our ammunition must be used with rental guns.
One gun: $8 / Sat: $10
All guns same caliber: $10 / Sat: $12
All guns (excluding carbines and S&W X-frame): $14 / Sat: $16
S & W 500 Mag or S & W 460: $30 each (includes 5 rounds of ammo)
Carbine and rifle rental $15-25 or $50 for all.
Full auto machineguns available by appointment – call for reservations.
Uzi 9mm: $50
M16 9mm: $50
MP5 9mm: $75
Did I mention I used to work there... so I do shoot free? No?