I am a little embarrassed to admit this, but I can tell youexactlyhow much they go for. The answer is $7.25, on average.

How do I know this? Well, for some strange reason (a moment of boredom perhaps) I decided to look at completed sales of 620 spools and calculate how much they were actually selling for. I found about 3 months of sales to average; and having done the exercise twice since then, I can now report the figure above as the average price per spool over the last ~9 months. It encompasses eBay sales of just the bare spools, and only those sold in North America. It does not include shipping costs, although I have statistics for that too. Anyway, here is a histogram of the data:

620 sales.png