I think it's possible to be a steal for both. Brian has the unique position of being able to leverage this business into something far more valuable than what he paid for it (a road that would have been much longer, difficult, and riskier for Josh), while Josh got a sum for the business that far exceeds typical super-early stage SAAS valuations. There are probably many owners of SAAS businesses generating up to, say $3k in MRR (obviously, exact numbers are impossible to pin down), that would be thrilled with such an exit.
It sounds like Josh also got to hold on to some of the upside while taking a bit of risk off the table. Seems like a win-win on the surface and with the experience Josh will get working with Brian, he may be able to grow his next venture to a later and more valuable stage. Kudos to both of them.