$100 may be excessive to some but not to all. There're people who earn a lot more than $100 an hour. I've ever attended a seminar where the presenter said that he was offerred a job that paid US$3.5K an hour but he rejected it cos it's boring. Would this be too excessive? It's not to the company.
There's no real need to justify. Prices are not determined on the basis of cost alone. It's also a balance between supply and demand.
It's not necessary to hire a pro for wedding; it's not even necesssary to take pics during a wedding. It's a matter of want. Some people wanted greater assuarance in the final results and are willing to pay the extra to get it.