I don't know exactly what the criteria is behind all this process, but what I can share with you is what I have observed and a little bit of speculation.
A tribe/token initiator would have told the scope and use case of the tribe/token while applying or getting registered with the Steem Engine. Steem Engine would have judged their claim (This could also depend on the reputation of the tribe/token initiator - not that rep, but what he/she has achieved/accomplished on the blockchain and what benefits they have provided to the community, number of followers etc) ... and the price is determined accordingly ... remember, this is a pure speculation. I have no solid proof to support this statement.
What I have observed:
When people start to buy the token/s, the price starts to rise as well, and when sell, price goes down. Many token owners are using it to artificially increas the price to attract people in buying the token/s. Some the token holders, who have a large number of token/s do this too in order to get maximum profit from their holdings.