You only pay of the improvements once.
You are paying to receive the card that then sits in front of you and you can use the abilities on it whenever they apply.
So If I put a worker on 1 major improvement spot I can buy major improvement (for example Well -> http://cf.geekdo-images.com/images/pic391245_md.jpg. I have to pay 1 wood or 3 stones). Then I can use it for free everytime I am on the move. Or do i have to pay for everytime i wanna play the card?