When you get to the stage where you have no cash left and there is no spice to be collected, send your harvester to the enemy camp. It will be destroyed, another one will replace it and you'll receive seven credits. You can repeat this as many times as you like.

Even simpler, you can destroy one of your full harvesters and you will find at least four times as much spice as you had in the harvester, scattered around on the ground in the explosion area. The lost harvester will be quickly replaced!

The following may or may not be true! To harvest spice in no time at all, click on a harvester, that is harvesting spice and keep clicking on it. The percentage of spice harvested goes up for each click on the harvestor. This save both time and money.