How do you determine operating cost of a 3 phase motor. 460 volt 15 amp.
My utility rate is .08 cents per kwh.
Is there a common formula?

Measure the actual amps drawn on each leg. Average. Multiply by 1.73. That's your amp load.

From there, it's the usual amps x volts x time x rate = electrical cost.

This will give you figures good enough to work with. Actual results will vary a bit, due to changes in load, power factor, harmonics, etc ... but the number you get will be close enough for budgeting.

One quick and dirty method is to estimate 1KVA of input power for each HP of output.

For a fully loaded motor you can use 1KW per HP. For lightly loaded motors use 1.1KW per HP instead.