retrieve list/menu label value ?
Ok, ignore the concatenate thing for now, it is generally cosmetic.
what i think you are trying to achieve is have a list/menu in your add to cart form, having the price as the value and the size as the label.
when you submit the form, you want to retrieve both the selected size AND the price, to pass into your cart.
if this looks like the thing, i'll take a proper look.