It is conceivable that a 50% rated VFD could be used just for starting purposes with little or no load on the motor other than that needed for acceleration. Such a VFD would have to be bypassed for full-speed full-load operation.
At these voltages, I'm not sure there would be any significant difference in VFD costing between the 50% load and 100% load units. The bypass would, of course, have considerable cost, as well.
About the best you can acheive with a reduced voltage starter is to reduce the inrush currents to one-half the across-the-line value. That would leave consider inrush current and the reduced voltage softstarter would have to be sized to handle this inrush current. On the other hand, an inverter could start the same load with no inrush at all, in most cases.
So, if you are soft-starting simply to reduce shock levels and mechanical stresses in the load, that can be acheived most economically with a reduced voltage starter. If you are trying to start with minimum inrush, then the VFD will give the best result.