A Question

So I'm giving some serious thought to republishing, as ebooks, two out-of-print short-story collections,  The Invisible Country and Little Machines.  Also, to compiling and publishing in ebook form a substantial new collection featuring the 'Quiet War' stories and some other space-opera stories.  How much would people be prepared to pay for ebook versions of a previously-published but OOP short story collection?  And how much for a new collection?  ('Nothing' isn't an answer, by the way.  I have meet the costs of publishing (new covers, formatting and so on); also, I have to eat.)