I think it can be read differently:
I still believe it is the shop keeper. However, you have to read it as if he is referring to Link as "Boy." I think most people read "Boy" as an exclamation, like God! or Crap! or Holy jumpin' Jesus!
Link walks in every shop looking to buy stuff that will help him along the way. Thus, he likely has the look of a hungry consumer. Due to the fact that the shop keepers of LoZ don't ever follow up with any other lines, we have to assume that this is the thing the shop keeper is saying to Link the moment he buys the ring. The shop keeper has a very expensive item, and he is communicating the value of the ring to the young boy buying it. If a little kid wanted to borrow your PS3, aspiring to take it from your place to his (and you were generous enough to actually lend your console), you might remind him: "Boy (or Tim or whatever his name is), this is very expensive." It's almost another way of saying, "Be careful."
Am I pulling at strings here? If Link doesn't stay a mute, my world will fall apart.