Yes, the value of a currency is based solely on the support and demand and the bid/ask priced on the open market. As for where it comes from, new steem is added at a set rate everyday and distributed as post rewards, this is often refereed to as the "reward pool" and why you see constant fluctuation to post payouts as it is always re-balanced against other peoples rewards. Why some say some users are 'draining the reward pool', hope that helps.