there are a couple other threads on this but i'll chime in again. any metal stain can be most easily removed with something acidic. you can rub lemon juice or vinegar on it, let it sit for a while, then throw it in the washing machine if you don't want to spend the money on an actual rust remover. there are tons of products you can buy at the grocery store that will do the exact same thing as vinegar/lemon juice but will also remove additional grease that may also be in the stain.
cheap: 1) dish detergent 2)lemon juice or vinegar 3) wash in laundry
cheap(ish): 1)go to the grocery and buy a pre-wash detergent that says "will remove rust stains"