I can certainly appreciate your concerns about the $300 charge for the Disney bagpiper. My father actually plays the bagpipes, and he occasionally does weddings here in Connecticut. But he charges $500 a ceremony because he's really REALLY good (I'm not just saying that cause he's my dad haha), so I just want you to be forewarned. I have no idea what other pipers might charge, but because a live bagpiper can be hard to find (let's face it, it's not exactly a common skill), they do tend to be fairly expensive.
I would suggest when you talk to your fiance, share your concerns about the cost of the Disney bagpiper, and then see about checking the internet for Orlando-based bagpipers. (Also make sure Disney will allow an outside piper!) If they turn out to be similar in cost, then evaluate just how important it is to your fiance, and if you both decide it's a high priority, try to find a couple less important areas where you can shave some of the costs to balance it out.
I really hope this is helpful, and just remember, whether you end up with a bagpiper or not, it's still going to be a really special day for you both!