As Max said; a responsive website that works on both mobile screens as well as desktop might just be a good answer. Assuming you did some research and based on your customer's need and high purchase frequency, you have decided that app is the right answer. There are other reasons to use app as well such as Notifications or using Apple Pay (thumprint pay - only for iOS though), might be your reasons to want to build the app. In that case you can save money by using cross platforms like Xamarin, which is a well seasoned and tested Microsoft tool. This platform will spit out apps for both iOS and Android. These work really well, and for simple eCommerce applications are probably the most economic option.
The exact costs are very hard to estimate without knowing the scope etc. I can probably provide some estimate to you, but I would need to know more about the overall business requirements. Feel free to email me and happy to engage further and provide insights.