Yes the price of the SBD, the falling post values ( I will go into that later one..) and the amount sent to booster per run defines the value. The decrease of the post value have even more to do with the vests which are in the market, so in times like now you could even see the post value goes down while SBD and Steem Price going up. Its complex :) If you are interested i can search for an article about, explaining it well..