Mjyeagle is correct. The modern lead-free solders cost more, but they are about 95% tin, so you get more of what you pay for. The balance of metals in these solders include metals like bismuth, copper, selenium, and, if you are really lucky, silver (which helps mold fill-out even more than the tin does). I've heard that some lead-free solders exist that include zinc, which you would want to avoid like the plague, but I've not seen it in plumbing solder yet. Since it raises surface tension, I should think zinc would interfere with wetting and not be suitable for plumbing, but keep an eye peeled anyway. Pick your product and get its MSDS before you buy, and it will tell you.
The Antimony Man has tin on his price sheet,
here.