Ignore retail price when adding to cart and only add option

Is there any way i can have my shop ignore the retail price when adding to cart ?

Currently I have my retail prices set to zero as the customer buys by weight which is set as options. The customer selects the weight required and this is added to cart if I add a retail price then the retail price plus the option price is added, i only require the option price to be added.

Thanks for any advice that anyone can give

Andy