Hi Andrew
I can make such script for you. I think best is to make php script which will be executed automatically, scan 3-4 different sources, compare prices and make json file. If any sources fail, it can send email to you that one of the sources fails. In this case you or your admin will know one of the sources fails and will have enough time to fix it, while other sources works.
I think making google as one of the sources is very good idea, as google rarely change design.
I will need to know structure of json file your app required, best is example json file if you can send one to me.
Payment on completion is perfectly fine, no milestone needed, you have more then enough reviews, I completely believe you.
Best Regards, Yura