Several years ago, a very important machine in an assembly plant malfunctioned, shutting down the entire plant. All of the company's technicians worked for days to solve the problem with no success. Finally, because the company was losing money with their plant shut down, they called the company who made the machine and asked for their best technician to come out and see if they could solve the problem.
A young woman arrived at the plant, and as she made her way to the machine, all of the plant's technicians watched her every move. She brought her toolkit up to the machine. Opened a very small panel on the front, and looked inside. She opened her toolkit, reached inside the machine and pulled out a very small fuse. She replaced it with a fuse from the toolkit and started the machine. Of course, it worked perfectly like the day it was installed.
She then turned to the plant manager and handed him her bill. He was irate. "You are charging me $4300 for replacing a FUSE?!!!!!"
"No," she replied, "the fuse is only thirty five cents. The $4299.65 is for knowing what was wrong and knowing how to fix it."
My point is, your contractor is a professional tradesman. It may seem like $75-100 is a lot of money to you, but you are buying a lot more than 18 feet of aluminum moulding. You are buying his expertise, his skill, and his experience. You are buying the fact that if the job doesn't look right when its done, that he will fix it so it does, even if it means getting more material. You are buying the opportunity to avoid cutting yourself, smashing your thumb, or damaging your new doorframe somehow.
I think it is important to value the services we buy from professionals. I give advice here because I enjoy it, and I enjoy the research that some of these questions make me do. It's not my job, and I don't get paid for it.
However, I am not about to go do work on someone's property for free. I am going to charge for my services, and the fee I charge reflects what you read above. Personally, I think you are getting off pretty good with that quote. I know some siding contractors who would charge a lot more.
Sorry, I'll get off my soap box.
If you want to DIY, go for it. All it takes is a trip to the hardware store to buy the RIGHT material, a way to get the material home, a few miter cuts in aluminum or vinyl, the ability to measure properly for the cuts, and the ability to attach the material to the home in the method appropriate to whatever material it happens to be.