Two easy ways of unlocking the "Stingy Bastard" trophy and getting max 999 white gems for purchasing red gems are as follows:
-In Act 3-2, you must wade through a little lake area chasing after Paula. In this area you'll find an alcohol dispenser. Three+ enemies will repeatedly spawn from this area, often dropping 2-3 gems after dying. To save on ammo, use the light shot and go for Brutal Kills.
The second way is in Act 3 via a demon. You meet him before the boss fight, just after meeting Colonel X. It is a teleporting Cyberdemon with sharp knives for arms and legs. Use the Light Shot on him so that he falls and freezes lying face down, leaving him open for a stomping. White Gems will repeatedly fall out of the demon, as stomping does not kill it. Do this to get the max number of white gems. This can be done in Act 4-1 and it also works with the ones in Act 3.