I have added purchase bills in quick books , but the cost of those items does not show for that items

I can help you get this sorted out, Zaheer uddin. 

We'll have to set this up on the Product/Service information, so it automatically show when you create a Bill. The cost will show under the Rate column. 

Here are the steps:

1. Go to the Sales menu and select Product  and services.
2. Select New.
3. Enter the necessary required fields.
4. Click on Save and close.

I've added screenshots for your visual reference:

For more details on how to set up product and service items in QBO, please check this out: Add product and service items to QuickBooks Online.

That should do it! If you have any other concerns about setting up items in QBO, please add them below. I'm ready to help.