The estimated cost to build a full-scale eCommerce app typically ranges from $80,000 to $300,000 or more, depending on the complexity of features, platform coverage (iOS, Android, Web), design requirements, and third-party integrations like payment gateways, CRM, or ERP systems. Advanced functionalities such as multi-vendor setups, personalized recommendations, real-time inventory sync, and compliance with standards like PCI-DSS or GDPR can further impact the budget. A detailed discovery phase is recommended to define the exact scope and provide an accurate estimate aligned with your business goals.