In the GameInfo Folder there are files for each Planet type; "Planet*NAME_OF_PLANET*.entity

Edit each one to adjust the amount the asteroids give in resources and other useful bits of info too. About 1/4 of the way down the files, you should see this(from PlanetTerran.entity):

planetResourceSetupInfo
asteroidSpawnAngleVariance 3.141593
totalMaxResourceAsteroids 3
metalResourceAsteroidSetup
minCount 1
maxCount 2
extractionRate 0.400000
refineryRate 0.060000
maxRefineryCount 3
crystalResourceAsteroidSetup
minCount 1
maxCount 2
extractionRate 0.400000
refineryRate 0.060000
maxRefineryCount 3
neutralMetalResourceAsteroidSetup
minCount 0
maxCount 0
extractionRate 0.000000
refineryRate 0.000000
maxRefineryCount 0
neutralCrystalResourceAsteroidSetup
minCount 0
maxCount 0
extractionRate 0.000000
refineryRate 0.000000
maxRefineryCount 0


Ignore the 'neutralMetalResourceAsteroidSetup' and the Crystal one, just edit the values of the top two. Should all be self explanatory, but if you are unclear about a value, just ask

EDIT: Well, it would look slightly different, there are spaces in front of the different sections making them easier to read in the file. But I can't for the life of me work out how to keep the tab spaces there in a post. ggrrr