well there is an obvious reason.
if you play the game and win prizes the chances are you'll need a swan gold, aswell as ablett ect.
there's about 10 golds that are usually needed each year to cover any possible prize wins and hold their prices until well into the season.
the rest are just golds and your standard $2-$3 jobs
same thing was with silvers when you needed them to collect prizes,
about 10 silvers always went for $5+ just for that reason.
s'pose it's easy now days anyway 'cos you can send in a b&f for any team gold,
be silly to send in the star/magic wild cards though.