hi, I am a full stack developer with 8 years of experience, and i have 7 developers team including me.
Is the for only android or iphone as well? The time and cost will depend upon your need, and technology we choose. I will prefer, react-native if you require both and good amount of performance. And if you are more concern about performance, you can go with native android/ios.
Your app will have 3 modules at least, mobile app, web portal, & backend rest api. So when you share design and features, we can make proper estimate.
We can discuss more over chat.
Regards,
Indra