The booking page on the KLM website will tell you.
Once you've selected the direct KLM flights and clicked on "Continue", you will see "Choose your favourite seat" under the "Extra Options" heading on the next page. Click "View Options" which will show you the current seating chart. Click on a seat in the Economy Comfort zone for each flight to see the cost. You can do this without having to first buy, or commit to buy, flight tickets.
If you have Flying Blue elite or elite plus status, be sure to log in to your FB account first, as you may be entitled to a discount.