I already developed a web app ( PHP, Code Igniter, Mysql plugged to json data ).
Inside the app, the user setup prices alerts and if the market rate goes up or down and hit the price alerts user previously setup, he receive an email telling him alert of the price has been triggered and this price alert is now disable.
As simple as that. What I need is plugging the feature push android notification ( GCM ) so user is also aware directly on his smartphone.
Happy to answer your questions!