I'm wondering because I noticed some books recently promoted at 20p, rather than free, and this seems to me a very sensible marketing idea! Much better than joining the increasing ranks of free books, which are getting rather boring and give a false ranking that does not count when your book goes back to the paid list.
I know we could just change the price for a limited period, but this would presumably not show up as a promotion, just as a cheap book, which puts some readers off... I also think the minimum price amazon allows works out at something like 77p (plus VAT).
Any ideas? Or is this something only traditional publishers can do?