The biggest advantage is the RISC-V architecture is open source, you're free to add or remove things (instructions) at your convenience based on what you're trying to accomplish. ARM is closed-source and licensed, while you can buy the technology via a license you get what they give you. Both are able to be integrated into your own hardware at the silicon level, unlike a CPU you'd buy from Intel or AMD. The reason the RISC-V stuff costs so much is because it's not expected that Joe Hacker will go out and buy it like a Pi, it's being sold to BigCo who want to develop a processor for their product that doesn't required license fees and can be right on the silicon with other components. The board is a ready-made development system, and it's designed so that you develop your application and then take what you've made and integrate it with your product.