Hopefully some collectors still have their receipts and can share what they paid and you can likely get an average price from that.
Like Kirby, Electrolux had a lot of bundle options and package pricing that factored in discounts and other things. You could also pay full retail if bought from a dealer's store. Door to door salesmen are more likely to cut a deal because they want to make the sale no matter what so people likely got theirs lower than retail.
When I make note of the original costs of the vacuums in my collection I just go with the manufacturer's retail price that was listed. Then I list what I paid separately from that.