Ehh, not really. As more nations grow bigger, big ones become even stronger, and the amount of donors remain constant, they'll be able to afford the donation more and more, and on top of that, it's still going to be a great deal for big nations (I'm at 6.6K, and it's still a good deal).
In short, with demand and necessity increasing on a steady pace with no increase in supply will inevitably make the price increase rapidly.
On topic, the deal is quite reasonable, in fact.