Unless you can find these lying around (I never did), this requires you to have $11,000. Try not to buy anything throughout the game and search for loose credits everywhere (you cannot sell anything for cash, unfortunately). If you end the game without getting to $11,000, you can start a new game+ where you keep what your full inventory in the new game, which should allow you to get the little bit extra you need to reach $11k (irritatingly, I needed <$100 when my game ended). Once you have $11k, go into your inventory and buy the cheapest plasma rifle ($6k) and cheapest RPG ($5k) to unlock the achievement.
You can always save the game before hand, get the achievement, then reload to get your money back.