Is "Payday" Money, real money?
In chapter 56 of the Pokemon Adventures manga, you can see how Kanto's Pokemon Fan Club Chairman had perfected the pay day move on both its Rapidash and Fearow. He used the coins to pay his ticket to ride the S.S. Anne, so they must be real coins. As to why Team Rocket does not use this method to gain loads of coints, it probably costs too much time and effort to be worth it. Imagine your cat could spawn a couple of cents a day and with some effort it might be able to spawn lets say a dollar a day. You might retain only a little after paying for the training and food to keep your cat healthy to spawn lots of coins. I doubt it would make someone rich quickly. I think Team Rocket would be much better off just robbing a bank or something or stealing from some rich dude instead of stealing or breeding a bunch of Meowth.