I have a homemade rotary converter that cost me around $50, including a used 3 phase motor. I have been using it for 40 years. I have 15 3 phase machines so it would have cost quite a bit more especially when you roll in replacements.I feel a rotary phase convertor will outlive cheap VFDs by a long shot. If the rotary phase convertor will last twice as long, then we are talking about more than 14 VFDs in the equation.
A couple years ago I added a Kay 10hp RPC that I now use to run my biggest motors and they do run much better mostly because my old idler is 7hp.
I would consider a VFD for a specific machine only if I needed the speed control feature.