For starters, the smartphone market has been such these days that there's nothing a 15k phone can do which a 7k phone can't. 15k phones will just do a few things better.
Let's compare the phones in these 2 price ranges based on different features:
1. Display: 720p for most 7k phones and 1080p or more for 15k phones. Though even 720p will suffice for most people.
2. Processor: 7k phone will have a slower processor but it will still be able to do most things which a costlier phone can do.
3. Looks: Subjective. Phones like Redmi 3s can hold on their own against more expensive phones.
4. Camera: a 15k phone will have a better camera. That's one place where manufacturers skimp to keep the cost down.
5. Battery life: no difference. 7k phones might even last longer.