Why was the Halifax death penalty reserved for thefts of at least 13½ pence?

This question is historically unanswerable. The wikipedia article is exhaustively sourced for this kind of article, and it didn't uncover the legal intention of the Baron who connived this law.

As wikipedia says, "Samuel Midgley in his Halifax and its Gibbet-Law Placed in a True Light,[c] published in 1761, states that the law dates from a time "not in the memory of man to the contrary".[6] It may have been the consequence of rights granted by King Henry III to John de Warenne (1231–1304), Lord of the Manor of Wakefield.[7]" The phrase, "not in the memory of man to the contrary" indicates that it was an ancient custom or practice. These ancient customs and practices were almost always inventions by one member of the peasant/lord power relation, usually as a way to circumvent other existing laws. As the Gibbet-Law was baronial law, it was probably an invention of John de Warenne or his ancestors. The actual practice in Halifax indicates that it was practiced in the class interest of the baron, "So strictly was the law applied that anyone who apprehended a thief with his property was not allowed to recover it unless the miscreant and the stolen goods were presented to the bailiff. The goods were otherwise forfeited to the lord of the manor, and their previous rightful owner was liable to find himself charged with theftbote, or conniving in the felony.[7]" We cannot know the prices of goods with any certainty from this era, nor the intentions of the law makers.

Wikipedia disagrees with my speculation above, following a speculation from Holt, "Eighteenth-century historians argued that the area's prosperity attracted the "wicked and ungovernable"; the cloth, left outside and unattended, presented easy pickings, and hence justified severe punishment to protect the local economy. James Holt on the other hand, writing in 1997, sees the Halifax Gibbet Law as a practical application of the Anglo-Saxon law of infangtheof. Royal assizes were held only twice a year in the area; to bring a prosecution was "vastly expensive", and the stolen goods were forfeited to the Crown, as they were considered to be the property of the accused.[22]" Holt, James Clarke (1997), Colonial England, 1066–1215, Hambledon Continuum, ISBN 978-1-85285-140-8 p23 and the following speculation, "But the Halifax Gibbet Law allowed "the party injured, to have his goods restored to him again, with as little loss and damage, as can be contrived; to the great encouragement of the honest and industrious, and as great terror to the wicked and evil doers."[23]" Midgley, Samuel; Bentley, William (1761), Halifax and its Gibbet-Law Placed in a True Light, J. Milner p404. I would describe these speculations as "whiggish" in that they ascribe to law an intention that was unlikely to have arisen, and give the law the character of an impartial disinterested power serving the community. We have strong and good reasons to believe that Baronial law never functioned with these intentions or character. Wikipedia cannot supply an intention or price of cloth at the formation of the law.

We do not have good wage-price series to 1304, and it is my real belief that wage price series before 1750 shouldn't be used. However, we do have a good to price equivalent from the last execution: 9s per yard of cloth. (Wikipedia).

If pushed, I would argue that 1s 1d and 1/2d is an intellectually pleasing amount indicating that it is one step more than a shilling and a pence: it is a compounded amount. I'd also dispute wikipedia's inflation of 1650~2008 £5.40, using purchasing power parity, and instead suggest something along the lines of 1270~2011 ~£13,400 using share GDP as a better expression of the economic weight of 13 and a half yards of cloth.

For the utility of the questioner: Wikipedia reports 9 yards (11.25 English ell) and 15 yards (18.75 English ell) stolen in the 17th century. 10 ell is 12.5 yards, 11 ell is 13.75 yards.

In 1779 the cloth trade traded 30 yard pieces ( http://www.thepiecehall.co.uk/history/ ). So if pieces were an ancient practice, the numbers have no relation to piece length.