Sub Grid - Quote products (Quote)

Hi Guys,

I'm having trouble customizing my quote form. What I basically want to do is have a sub grid whereby I can select one of my products and add it to a quote for a customer. 

Please see this video on Youtube, as this guy is doing exactly what I want my form to do: https://www.youtube.com/watch?v=0yPDawJ1hlU

At present the grid will let me select products, but the information isn't automatically calculated in the "Detail amount".

Note: when I add a product to the Quote products (Quote) sub grid, a yellow highlight appears at the top of my page with the information = "! the Following error occurred: An error occurred during pricing of detail line item."

Any help / advise please?