If you use the default Dialogs, you have a choice of 3 icons to use: Quest, Help and Shop. One of them, the shop, shows a dollar sign. However, this is no longer shown, and even gives an error when trying to load one:
“Image failed to load CoreGui.ChatNotificationGui.Background.Icon.Image : rbxasset://textures/ui/DialogShop.png because File not found”
I believe this icon is missing from the assets in the current client.