How to price an ebook based on existing blog posts?

Value value value. If there is value, charge for it. The Martian used to be blog posts. Then he made it into a book. Which got made into a movie.

Don't discount it because some of the content is available in blog posts.

In internet marketing circles people point to some magic numbers that usually works.
Like the best basic number is around the 19.95$ (sometime 14.95$)
than every upgraded product/up-sell can be X3
So people that buy the 20$ have a good continuance to buy a 60$ product and these to like 180$ product.

Don't forget you can always make discount campions / sales for less serious / cheaper buyers... But taking a price higher is a bit more costly / less efficient than reducing it, so if you are considering multiple price points, start from the higher one.

If you got a traffic source and can stagger the offers, you can a/b test different price points..

As much as I know all of these tricks, and I'm a cheapskate which doesn't buy much, I bought a product on a year sale for 7$ which is retailed for like 200$ (and had different discounts over the year..), I've never used it or asked for money back. I opened it once, looked at the titles and that's it.
90% of books bought are never opened. 95% never pass the first chapter. Way less than 1% of books are ever read cover to cover.
If you could, to maximise revenues, you would price it differently for every person.
But pay what you want for example doesn't lead to that maximisation.
It sounds like the best path is just start high and randomly discount at different times.