Different Costs for Different Tasks

I would like to see the ability to give different tasks in a single project different cost rates. For example, any inital design work I put into a project, before the project is confirmed is effectively done for free. However, I would like to bill this as a 0 cost item against the client, so the amount of work performed is visible to them.

Example:
Sample Project
Task - Brief & Inital Design - 12 hours - £0
Task - Inital Design - 6 hours - £120 ( 6 x £20/hr)
Task - Implementation - 12 hours £360 (12 x £30/hr)