I recently received an email requesting more information and a clarification on my previous posts. I thought it might be helpful to post my reply here, also, in case I was unclear or others might desire an encapsulation:
First, a bit of a correction. The 2 1/2 point blanket is sized more along the size of a modern twin bed blanket. I'll check the actual dimensions and send, but I'm sure you have a number at the fort (can't remember if you have Rob Stone's among your blankets; his are well sized and used by many of us). So they aren't 12'x12'. They were woven, by individual weavers (some with multiple workers), on looms in varying overall sizes depending upon the type and size of blankets being called for. The Alfred Plummer books, "The Blanket Makers 1669-1969: A History of Charles Early & Marriot (Witney) Ltd" and "The London Weavers' Company, 1600-1970" are two good resources, among a number of others I have used.
The blankets were taken in large batches (I forget the term) to the Hall for guild approval before being cut for shipping, usually into double lengths of the blanket size ordered. The blankets were shipped in the double lengths, hence the term "pair of blankets". The whole sale price (which I am trying to track down from mill to sale) is based upon this double blanket (price paid to weavers was on overall length woven or weight).
On the price, 8s. is indeed an "average" wholesale price for an individual blanket (one half of the double blanket sent as a "pair"). This is based upon the Grand Portage inventory, the Grant Campion invoice, and other price documents. The 8s. is considered the "cost" (wholesale plus overhead--canoes, men, food, etc) to get that blanket to Grand Portage, and most likely by extension Fort William, though the price and cost was higher by the time period you portray there.
On the plus, I thought I had included some of the documentation sources. Mainly, the references used had to be secondary (Innis "The Fur Trade in Canada", and Parker's "Emporium of the North") because I don't have access to the records they have/had. Those that were at the MN Historical Society have disappeared.
However, the primary documentation they cite seem to agree both on the varying price structure for the time period I am viewing (1780s to around 1800) and weight of the pelt itself. Ross was one of the cited primary sources. The actual prices varied widely (and wildly) in this time period making comparisons that much more exciting.
Using the records compiled by Harold Innis “The Fur Trade in Canada” and James Parker “Emporium of the North”, beaver brought 8/6 to 10/2 (1784), 15/6 (1789), 1802-5 average 14/. A. Henry in 1806 mentions a beaver pelt being worth 10/ Halifax.
Two pounds for the weight of a pelt seems to be the consensus average, thus the 3 to 4 plus "price" for the 2 1/2 point blanket times the particular price for peltry at any given time.
1 beaver pelt worth 17 to 20/4
Return on one 2 1/2 pt 46-55/ Montreal
1 beaver pelt worth 31/
Return on one 2 1/2 pt 88/
1 beaver pelt worth 28/
Return on one 2 1/2 pt 79/
One thing to keep in mind, most of the blankets were not sold at either Grand Portage or Fort William, so the overhead was much higher, almost prohibitively so by the time it reached the Athabasca Department. Parker's "Emporium" has some good information on this. No matter the overhead (and the Wallace documents gives a rudimentary idea with the advance percentages charged clerks in their minutes on partner agreement regarding prices to be charged) the Native trading partners wanted the blankets for the customary price of 3-4 pelts. So going west, profit became problematic.
Let me know if I am still lacking clarity. You can post here or reach me at firstname.lastname@example.org