Photography disagreement

I definitely think you're right and photography is something you want handled correctly. Yes it's quite expensive, but the cost is usually justified by the amount of time put into your photos by the photographer. It might help to break down the cost to your parents that way to help them understand why photography is so expensive. They spend up to 12 hours at each wedding they photograph and then spend hours sifting through the thousands of photos taken and editing them. Plus they have good equipment - lenses, cameras, editing software, computers, etc. Sometimes they also pay an assistant or second shooter and often an engagement shoot is included. So at the end of the day it's not a huge stretch to pay thousands of dollars for a wedding photographer. You obviously know some of what goes into it being a hobby photographer.

What sort of range have you shown to them? Is there anyone you like on the lower end of things that you would be happy with and they might go for? If you absolutely can't come to an agreement with your parents I would pull from somewhere else in the budget to get a decent photographer.

Edit: Another option would be to see if you can hire a professional but for a shorter period of time and then have these friends of friends/amateurs photograph the less important parts of the day.