Honestly, I think it depends on whether or not a price drop is a good thing.
While under the assumption that a price drop is a good thing (as in the case of a store's products going on sale), perhaps green is more appealing to the customer.
On the other hand, red could be more appropriate if the action is bad. Using the example from Chairman Meow, a decrease in stock prices means a loss of money to those who own the stocks.
I'm not sure what the purpose is of your price tracking, but if it is solely to gather and display statistical data, then I would go with red. Since there is no implication of good or bad to viewers, a decrease (in itself) is a negative action.