With these levels, I might assume that the speeds aren't particularly fast. (See Footnote.)
One simple method:
Each input requires a 4.7 Volt 1/2 Watt Zener diode (1N5230 or equivalent) and a 2.7K Ohm 1/2 Watt resistor.
Input signal goes to one end of the 2.7K Ohm resistor. The other end of the resistor goes to the Arduino input pin. The cathode of the Zener diode also goes to the Arduino input pin and the anode of the Zener diode goes to ground.
With zero Volts input or open-circuit input, the Arduino input pin is at logic zero.
With 12-30 Volts input, the Zener diode limits the Arduino input pin voltage to something less than 5 volts. The current through the resistor and the Zener diode is something between 3 and 10 mA. The maximum power dissipated by the resistor is about 0.27 Watts, and the maximum power dissipated by the diode is about 0.05 Watts.
Piece part price of the Zeners in quantity 100 will be probably be less than $0.10 USD
Piece part price of the Resistors in quantity 100 will probably be less than $0.05 USD
Regards,
Dave
Footnote:
Before buying tens or hundreds of parts and fabricating tens or hundreds of interface circuits, I would recommend that you build and test one or two prototype circuits to make sure performance is adequate for your application. If you have a lot of 30 Volt inputs that stay continuously high for a long time, overall heat dissipation could be an "issue."