After creating a Custom Object and adding lookup field to a Case object, Case's detail page can have a related list of this Custom Object. This Custom Object has a field Quantity. I need to put 'Total:' under this related list. I tried insert VF page - ugly - constant height of section is killing everything. Beside this, after clicking 'Go to list (xx+) >>' it opens a new VF page with headers and side menu - ugly. Create a VF page for Case and put Related List to the bottom - no links, on which you on hover and your related list is show right under your mouse, without scrolling down or jumping to it. Any thoughts?
If your custom object is a Master-Detail object from the Case, you can add a new field to the case as a roll up summary which would be the sum of your quantity field.
If your custom object is just a look up field to the Case object, you can create a new field on the Case object. I would then create a trigger on your custom object and whenever the quantity changes, update the total field value on the case.
If you want it all in one block, you can create a Visualforce page and embed it in your page. You can remove the header and sidebar in the <apex:page>. If you're having height issues, I would recommend using the standard <apex:pageBlockTable>