just to note the only thing that counts are the items under the Upgrades tab,
upgrading weapons, buying books or special etc do not count.
To maximise your Gem per match purchase the Bigger Gem Pile upgrade, equip the Santa's Little Helper weapon and the Wealth book. these all increase your count.
To get massive sums of gems visit the map and choose a stage with a ? symbol or a boss Stage, these are bonus stages, remember you can also replay each of these levels on a different setting. These particular fights pay out far more than regular fights.