Free Sources: We go out in the desert to the popular shooting spots with buckets and shovels. The outdoor range where we live won't let us scrounge for lead, but occasionally one of the guys at the local indoor range will sneak me a 5 gallon bucket.
Cheap Source: The local metal scrapyard will sell lead of unknown alloys for 25 cents per pound under current lead market rate. He also sells pewter/tin of unknown alloy for the same. I get most of my tin as pewter figurines sold at the local second hand market for pennies of their actual value in tin.
Then to actually create my alloys, I cast an ingot from a big pot of range scrap, test the hardness, then run it through the lead alloy calc to determine how much of RotoMetal's "Super Hard" (lead and antimony alloy, no tin) to add to get my desired BHN. I'll also usually add 4% tin, too, but depends on the estimated tin content of the scrap I'm alloying.
I'm able to get enough free lead that I don't have to pay for it. The tin is fairly cheap, but I do end up paying for the little pewter figurines. The most expensive part for me (and it's really not too bad) is the RotoMetal's "Super Hard" that I use to get the alloy I want.
We also keep all the copper jackets from the range scrap, along with any stray brass we collect at the popular outdoor shooting spots. We then sell/trade this to the scrap yard in exchange for either tin or lead. They always cut us a better deal when we trade metal in in exchange for another metal than when they actually pay us in cash for the copper and brass.
I haven't put an actual dollar figure to it.... I probably should, but the system we have of scrounging for lead, getting the odd free bucket of it, keeping the copper and brass then trading it in for tin or lead has made our shooting alloy very close to free.