So, I did discuss this a bit with another player as well, but it feels like the dummies at the Font give far too much coin for being there for training purposes, so much in fact that players can end up with hundreds of platinum and never have to leave Waterdeep, which does feel like throwing off the economy quite a bit. Not even goblins give that much coin. So my suggestion is taking out the gold/electrum from the dummies, making it so they only drop copper/silver. There should be some coin still for training with the dummies as most do, but dummies in general should be brought to a standard drop rate of coin that isn't going to outshine live mobs that would actually hoard coin.
Those poor Ohgmans, I'm sure the Font is deep in the red with all the coin they pass out.