We can also look to published accounts about prices for quilting---secondary sources.
In 1920 Emmett Leroy Shannon wrote a book on how to earn a little money on the side. Money for the Woman Who Wants It had a few paragraphs on quilting for pay. "The old rate...$1.00 per spool." He explained:
"The patron furnished everything, thread included, and the quilter had $1.00 for the work required to use up a single spool....Some will take three, five or even seven spools, according to their design."
Tying a quilt about 1910
In the early 1920s Shannon thought a fair rate might be $2 or $2.50 a spool, while the ads of the time indicate the going price was closer to $1.
Searches for quilting thread prices haven't yielded much information.
This 1937 ad from a Marion, North Carolina department
store gives some prices.
"Quilting thread, 4 balls for 5c"
This may be a thread locally produced in a North Carolina mill and sold by the ball rather than the spool, perhaps that thick thread often seen in Southern quilts.
Maybe the 1970s
Linda Degh's Indiana Folklore: A Reader, published in 1980, included an interview with a Mrs. DeVault about the economics of quilting. She tells us the standard $1 a spool story in the past and then explains, "She would not quilt for less than six cents per yard."
A quilt [requiring] at least 800 yards of thread = $48. "It takes three weeks to do it."
Let's hope professional quilters are recording their prices today.
And that's the end of this thread.