For the most part, it seemed pretty unpredictable, and I did keep notes on my attempts to try and discern any rhyme or reason to their certificate pricing. Only two observations - going west to east from LAX there was more 1-cert inventory than east to west, and any flights where the Mint cabin was more than half full already did not offer 1-cert redemption (and these were often the more expensive cash fares.) I did have one case (the return flight EWR-LAX) where on initial attempt it showed 2 certs, and then when I checked again 2 weeks later it had been reduced to 1 cert. Otherwise all other routes I checked after a time delay maintained the same certificate pricing.