The latest issue of Folk Leads, available from Sam Dodds on ed.sam.music@virgin.net, has an article on Sam/Jack Hall, the song, and related songs. Jack Hall was a real person, hanged, according to Roy Palmer and the Newgate Chronicle, in 1707, having been previously condemned in 1701, but then let off for unknown reasons--possibly some government initiative on crime. A 'bull', according to Grose's Dictionary of Slang(1811), was a crown piece(5s.=25p), and half a bull half a crown, but I've been able to discover whether a bullock was the same. But as 'Sam Hall' has 'twenty pounds' to Jack Hall's 'twenty bullocks' it's possible, and if so would have been 50s., a reasonable amount of savings then.