When casting I often alloy my own metals, it saves quite a bit of money. When buying casting shot they put in some trace metals that are suppose to make a better cast. So when I cast gold I buy gold alloying shot & use this cart for the percent of the gold, last time I priced gold shot for casting it was about twice the cost of the gold in the metal. Sterling I go to a local refinery or coin collecting store & buy fine shot or a cheap coin (they run about $.50 above spot price, which is the trading price from the day before), and go to the hardware store to buy the copper. I use to work at a bronze cast company & just before casting we would throw in the tin otherwise the tin would burn up in the melting process, so make sure you look at the melting points. 

For weighing the metal I recommend a reloading scale (get this at your sporting goods store, it is used to weigh gun powder), but the new digital scales are coming down in price. I had one salesman tell me to go to a police action & buy a scale from a drug bust, back then you could get a triple beam for $15, to check the accuracy throw a nickel on it (a nickel weighs 5 grams).