Default values for attributes

Hi guys

I have a simple website selling flowers. Every product is available in small/medium/large and the 3 size prices are the same for ALL products… apart from 5.

Ive added my sizes to the Attributes screen. But it seems I can only add the corresponding prices for each attribute on the product details page. How can I have the prices apply to all products by default, and then override these on the few products that differ?

Thanks