The stated goal for a Neptunium game is "The winner is the first player to accumulate 200 kilos of Neptunium..." But in short time I've been playing, neptunium plays a relatively small role for most of the game time.
I believe in order to really enjoy the idea of racing to get neptunium you need to enable drillers to earn neptunium while stationed at an outpost, without having to build a mine.
I'm not saying to abandon mines altogether, merely allow drillers to earn a smaller percentage of neptunium while stationed at an outpost. The player would then have to weigh using drillers in combat vs. using them to mine plutonium.