USB isolators are going to be expensive because of the high frequencies involved, it would probably require and USB transceiver on both sides of the transformer. If the USB is converted to something like UART or SPI inside the device, it's much easier to isolate that. I like the RF isolation devices like the AD iCoupler and similar devices from Silabs. There is a USB device in that line, but it's expensive and only does USB 2.0 full speed (which ironically is not at full speed).