I think how much minerals you get is proportinal to the amount the user has and how much money you did to him i.e. I'm ranked like 30 because i have infantry and canister rifle. I attack some guy like you that has 300 something minerals and is in the bottom 30 (the bottom 30 are all the same, 10 troops, no weapons) and i get 4 min per attack when I do 421 or so and he does 21. When I attack another guy around there I'm lucky if I get one mineral. Also, spies can attack weapons and make them a lot weaker (if it works.) Then you can attack right after that and own them
. Also, I'm not sure, but I think sentrys are anti spy. If you have a sentry and they have a spy they can't see your mineral.