It's not an easy question, since it doesn't have a clear-cut answer. Development costs depend on a great number of factors, including app complexity, functionality, app type, number of platforms, etc. For example, let's have a look at the must-have functionality of on-demand apps:
- User registration and profile (Design - 10 h/BackEnd - 24 h Android/iOS - 40 h)
- Map integration (Design - 4 h/BackEnd - 16 h Android/iOS - 8 h)
- Filters and lists (Design - 12 h/BackEnd - 16 h Android/iOS - 40 h)
- Vendor profile and service page (Design - 16 h/BackEnd - 16 h Android/iOS - 32 h)
- Shopping cart and payment gateway (Design - 8 h/BackEnd - 16 h Android/iOS - 24 h)
- Push Notifications (Design - 2 h/BackEnd - 40 h Android/iOS - 32 h)
Therefore, it might take 8 months of development and cost from $70K per mobile platform.
You can find more information on on-demand apps here.