what is the easiest and cheapest way to improve the sound system on a 3 Gen Bose stereo. Give me some figures please. Remember, my pockets are thin.

 

If you want to leave the Bose in, add a subwoofer. Like you said your pockets are thin so take a peek at Bazooka Tubes. They provise enough extra sound and rattles for you.

If you wish to remove the Bose adn upgrade, you are going to need more money.