The lead isotope assaying is done by a standard Lead Isotopic Ratio Package. Isotope ratios are measured using mass spectrometry, which separates the different isotopes of an element on the basis of their mass-to-charge ratio.
Just because it is from a shipwreck, does not mean it is low alpha. It may have been mined in an area with a high radioactive background. Remember, lead is depleted Uranium, so they are frequently found in the same locations. Radioactive dating measures the uranium vs lead, Pb206 is the final decay chain of U238, with a half life of 245,000 years. Pb207 from U235 and Pb 208 from Th232.
As of June 2012, depending on the lead isotope Pb204, 206, 207, or 208, the value ranges between $25 and $100/pound. (not in any order, but depending on the isotope)
Pb204 is primordal lead, with no radioactive isotopes, standard lead is Pb208
It is difficult to refine to keep the contamination from affecting the isotope value, hence the extremes in value.
Good luck!