A Price comparison site that compares prices from various famous website and show it to you for better price purchase from the list.
If you want to build something similar like junglee.com, mysmartprice.com, etc.
You can achieve it via a technique called Scraping.
You can scrape the price, description, reviews and other details by setting the dom elements.
Than you can store those scrape data, and set a cron job to run those script for min. 1 hour and then regularly update it to show real time price.
You can earn money from affiliate site of the site involved in comparison and also once the traffic of the site is increased, you can earn money from google adsense from advertising on your website.
Let me know how did u liked my idea.