If you want to run your business, generate sales or acquire new customers through an app or a website, it is most likely that a ready-made product is not going to be enough.
Individual price offer will help you get an idea about the price for your mobile or web app. To get a precise calculation, your developer will surely need to know quite a lot about the project itself. You should have a very precise specification ready to speed the process up. Even though you can still compare individual offers, compare the hourly prices, hourly testing budget, you can't actually predict the final price and your satisfaction with the developer.