Why do my group items have a $0 cost?

Welcome to the Community, razimov.

All products that are added to group items already have cost. This is the reason why your group items are showing zero.
 

Once you add this item to your invoice or any sales forms, you'll see the individual product cost.
 

To know more about how inventory works in QuickBooks Desktop and this will affect your Profit and Loss report, check out this article:

Understand inventory assets and cost of goods sold tracking.

FIFO Options in QuickBooks Desktop Enterprise.

Don't hesitate to comment below if you have other questions with QuickBooks Desktop. Take care and stay safe.

@razimov 

RE: but when I combine them into a group, it [the group] shows it as $0 for the cost column.

Yes, as @Rustler mentioned, a group item has no cost in and of itself. It's just a container to let you quickly paste a bunch of items on an invoice.They have no cost: you can't buy them. You can only buy the items in them. Groups were around before QuickBooks even had an inventory feature and just aren't intended to show the cost you're after. It would be better if the cost columns on the item list was just empty for group type items.

What works more like you want is an Inventory Assembly, which will show cost. Assemblies are much more robust, but also has some overhead you may not want. You have to "build" assemblies from other items in order to then sell them, for example, which removes those items from inventory as if you used them up.