Building an e-commerce app involves several steps: planning, design, development, testing, and launch. Costs can vary widely depending on features, with basic apps starting around $25,000 and advanced ones reaching over $80,000. Must-have features include user accounts, product browsing with search and filtering, secure checkout with payment gateways, and order tracking. You can also consider adding trendy features like AI recommendations and AR/VR product views to stand out.