Psyringe
Scout
Yet I bought Civ Bucks because they are cheap and compared to Zynga Cash or other established FB virtual currency they provide you with a huge benefit and therefor it is value for money. Firaxis did a great job restricting each player to use just 10 Bucks per 24 hours. Imho this is great for the game balance as rich people can not dominate the game and win just by their power of money in the real world.
This sounds like a pretty shabby method of getting into the players' pockets. If what you say is true, and the "bucks" do make a huge difference, then players will need them to stay competitive. This means that the "free to play" slogan doesn't make much sense, it's not free to play in practice if the player's choice is just between either paying or losing.
The system that you describe seems designed to:
1. Lure players with a "free to play" slogan.
2. Let players realize how important the "bucks" are.
3. Facilitate a low spending threshold in your players due to the low price of said "bucks".
4. Give the players who keep spending their money an illusion of "fairness" by limiting the amount of bucks that one player can buy in a given timeframe.
5. Probably (this doesn't follow from what you wrote, but would fit very well into the system you describe) encourage players to bring their friends into the game, and to persuade these friends to spend money themselves, because the per-person spending limit makes groups more effective.
It would probably work economically. The one thing I don't understand is why players allow themselves to be milked so easily. Players will end up buying bucks regularly as if the game was offered with subscription fees - though asking for subscription fees would at least be more honest than first luring players in with a "no charge" premise, and then designing game mechanics that require people to spend more money if they don't want to be left behind.