Quite honestly I'd let the market battle it out for "value" by listing it at an auction like Flippa.
Without having the need to purchase a domain like this, if I were to think about the brandability factor, it isn't very high because there are so many other letters that can be used in a similar fashion: Josting, Kosting, Mosting, etc. So it'll come down to if someone really wants "Zosting" and nothing else. At this point I'd place registration value.
If that was a typo domain like Gosting.com with some traffic, obviously the value would positively change.
