I don't believe there is a set minimum word count or a "standard". Generally speaking on the low end you'd see something in the 15-20k range. Naturally the smaller the book is the less a consumer expects to have to pay. Instead of focusing on word count I would focus on my message goals and how to help the reader improve in the topic area.
On the low end you see eBooks selling from 99 cents - $2.99
My own book had slightly over 40k words but it was a print book that was made available on Kindle. Best wishes!