Cross-Platform Ecommerce App Cost

I want to estimate the budget for developing a cross-platform ecommerce application before contacting agencies. The project would include iOS and Android support, authentication, product browsing, shopping cart, checkout, payments, push notifications, analytics, and integration with an existing ecommerce backend. Which factors have the greatest effect on the development budget, and how much does cross platform app development cost for a project with these requirements? I would also appreciate advice on separating essential features from optional ones and planning a realistic budget for future maintenance.