Answer:
There are more letters available for collectors to buy that were written by Lincoln than there are letters that were written by Booth.
Explanation:
According to the law of supply and demand, the price of a certain good will increasing if the supply of that good decreased.
Since there are less letters written by booth, the supply for that particular goods also decreased, making that good really scarce in the market. This is why the price of letters by John Wilkes Booth is higher than Lincoln's.