Definition from this site ５分と５分の感覚。白黒両者が不満がない。
A sense of 50-50. Neither player, black or white, is dissatisfied.
Bob Myers: Pending some of our experts weighing in I will comment. Souba means "market price", most often in the context of equities, commodities or currency markets. Its use in go is as the original commentator wrote, although the term could also be applied in a wider context, including describing a move actually played. If forced to choose a single word to translated souba, I might choose "reasonable (outcome)".
Bill: Thanks, Bob. I have been searching the web for examples, but have only found generalities, such as Go is souba, The Japanese style of go emphasizes souba, and This game is not souba. From some of what I have found I have gotten the impression not of best, but of not bad, or not greedy; or, as you say, reasonable.
Bob McGuigan: In my experience the term is used when explaining how things should turn out if mistakes are/were not made, so I'd pick "fair" as an English descriptive term. I don't mind "best" in the sense of "given the starting situation, with all it's flaws, this is about the best both sides can do" i.e. a result that is fair for both. This is consistent with "reasonable" and "market price", but these quibbles are only important for translators aren't they? I doubt that souba will be taken into English as a go term.
John F. I discussed this term in an article on Wang Xi in New In Go, item 20. Sometimes I translate as "par".
Bill: Thanks, guys. The New in Go article talks about souba as a style of go.
RobertJasiek: So is it not a term but more like a frequent common language expression like "fair", isn't it?
[Z_]: Could the meaning of souba be something like when I win an ebay bidding, and I am neither happy nor sad about it at that price, it's par, no gain, no loss. It is souba, fair price, fair exchange, nobody gains nor loses considering their circumstances, unlike when I win at a raffle, when I'm happy, I got more than I sacrificed, but it was souba initially during the purchase given the "odds." Or when I have to pay insurance prices mandated by law, or utility monopoly minimum charges, and I feel bitter, as in not souba, not fair open market or free market exchange of values under the given circumstances of the participants mindsets.