There is a limited number of Green Nodes that exist. As Nodes are purchased, that limited number will decrease, and the overall cost of the remaining Nodes will increase.
When you're in the checkout process, you are given one hour to complete the checkout. This is holding the price until the transaction is completed. If nothing is sent/received from the particular blockchain, then the transaction will expire. If you try to go back in and re-purchase, you will be subject to the new pricing.