To put that into perspective, reports I read put MIPS at $700-900k with ARM starting at $1+ million. Buyers of those get their ecosystem of existing tools, OS's, libraries, etc. RISC-V doesn't have much of an ecosystem yet. However, if FOSS software covers application requirements, then a SOC using the RISC-V chip saves a lot of money upfront. Even more down the line since SiFive doesn't charge royalties.
(Edited to change part about royalties after reading article more closely.)
From what I understand, the RTL of these cores and a good chunk of the stuff around them is open [1]. The cores themselves are basically all specialized Rocket cores. What you're paying for is probably the ready-to-fab routed cores in whatever CAD format and support.
They charge for the peripheral IP and system design. The actual computation core architectures are freely available on GitHub under open source license.
(Edited to change part about royalties after reading article more closely.)