they're pretty cheap, under $300. You can't run 3hp on a normal 110v outlet which is why they don't really make VFD's that do voltage conversion above 1.5hp since that is the limit on a 15a circuit. You MIGHT be able to squeak a 3hp motor on a 20a circuit, but not when you factor in the losses from the VFD doing phase and voltage conversion. Anything bigger than that is going to be 230v in. The way I see it with VFD's though is that they are close enough in price to a motor starter, that you are far better off getting a 3p motor which is more efficient and longer lasting than a 1p motor *and often cheaper*, and then spending a negligible amount of extra money on the VFD where you can control starting and stopping, as well as fine tune the speeds as needed vs. just buying the motor starter. For a normal 5hp compressor, a soft starter is going to be about $350, where the VFD is $550. Not a lot of extra money given how much more versatile the VFD is and not only that, the soft stop really helps in motor and compressor wear as well.
On the 90%, that was a typeo, it's typically around 80% IIRC. Some of the oiled compressor guru's may be able to comment, but it has to be around 80% since the European equivalent motors running on 50hz run around 83% the speed of their American cousins running on 60hz, but they don't like to go much slower than that.
If you put a VFD on a Rix, also remember that you are only going to be able to go down to about 20% of the rated RPM of the motor or you'll have cooling issues with it and they get pretty angry, so any slower than 20% of rated nameplate will have to be done with pulley changes or using a slower motor RPM *which is more expensive, so do what you can with pulleys before trying to go to higher pole motors
