The cost depends a lot on the horsepower of your machine. Ebay has lots of phase converters for sale. You need a realistic approximation of the true power draw of the machine including spindle, coolant pumps, and hydraulic pumps. So you might have a 5hp spindle but there's a 1hp coolant pump in the base and a hydraulic pump that uses 1hp. If you go too small on the converter, whenever you turn the coolant pump on, you can trip a circuit breaker.
Generally, the rotary phase converters are better than the static phase converts. Obviously, the rotary converters are also more expensive. Both my VMCs are running on rotary converters.