There are a lot of factors at work here. One is location. This appears to be the inventory of a gunsmith in a northern colony, otherwise the carpenter trained slave would have a much higher value.
One must take the fluctuations of currency value into consideration also. Money does not retain its value consistantly. I know that during the Jacksonian era(1820-1850) the average cash income was around $12 a year. That is not total income, since trade and barter was a large part of the economy. Cash was scarce. Spanish and French coin was legal tender until 1838.
I have seen several gunshith account books that listed the value of a new rifle at $17-$20, a smooth rifle at $12, for new work, soon after the revolution.
Inventories at probate were often inflated figures, valuing furnature and clothing at more than it's value when new since they were being done for tax of inheritance purposes.
I know that on the middle ground frontier, middle TN espically, there were more guns than there were people. Smoothbore muskets were valued in cents rather than dollars.
By the beginning of the 1800s there was no shortage of rifles. They were being mass produced in the shops of Lancaster. Thousands of pattern rifles and smoothbores were being produced for the Indian trade. Citizens were gripping about the polution of the waterways by the barrel grinding shops.
Supply, demand, peacetime or war, availability of raw materials, availability of riflesmiths, they were all factors on the everchanging price of weapons.