If you add those islands and tag on the Big Island, I would not be surprised for the cost to reach and exceed $1.5 billion. I would, though, increase the power handling capacity of the cable to at least 350 MW, and 500 MW would make more sense, as the average power used in the state is around 1250 MW.
As we all now know too well, the electricity bill for Hawaii is already three times the national average. In time it will be 4:1, until sustainable options kick-in, and a statewide electricity grid is essential to energy self-sufficiency.
The challenge would be depths approaching 7,000 feet. Ratepayers almost surely will pay for this expense. Kauai is never mentioned because you would need to traverse nearly 11,000 foot depths. In time, perhaps Tesla's tower of power (above) or the technology associated with solar power from space (below) might be considered.
I'm certainly not advocating those fearsome (I show these graphics with purpose, as you will see) alternatives for Kauai, but I wondered if it would be sensible to provide a future link to accommodate OTEC floating platforms? The OTEC prime sites (the redder, the better) are located south and west of the state (from Gerard Nihous):
What are the relative economics? As a 100 MW OTEC plantship could cost around $800 million, the order of investment is very similar to the proposed $700 million connection between Oahu and Maui.
Hurricane Ingrid in the Gulf of Mexico, at 75 MPH, is just about ready to crash into an area just north of Tampico, Mexico: