Hi, I am tryng to set up certain items that when they are bought you can have a certain amount of free items.

For example, with a bike that is worth like £400 I wanted to give 10% worth of free items of the customers choice ( so £40 of free goods). How would i do this, i guess you would somehow do it in attributes? Would this allow a customer to add £40.00 worth of free goods without it being added to the shoppng cart bill as £440.00? Thanks for any advice given.