The least expensive casting metal is pure lead, which I purchase from Roto Metals in 5# bars. To harden it, I use their Lino in 5# bars. Mostly, I mix them 3-1 (pure to lino) and get a alloy that is near to WW alloy. Never found the need to add tin, either. Measures 14-15 BHN after three weeks, air cooled. If you need harder, you can oven heat treat.
With pure and lino, you can mix at other ratios............... 5-1, 4-1, 2-1, 1-1, depending on how hard or soft you require. Or whether or not you want to heat treat. It's my KISS principal.