First of all it isn't "only" a porch. In custom renovation there is no "only" anything. It is all about details--studiously conceived and finely crafted details. All those aesthetic accents you admire when you browse through a home design magazine? Those take time and care to produce. A porch addition we recently completed in Chevy Chase DC illustrates this idea quite well.
Our architect drew this addition to match the architecture of the turn of the century (that would be 20th century now) main house. A key feature of the house is the decorative rafter tails.
Why does it cost so much? The more drop dead beautiful it is, the more balanced and nuanced it is, the more you love it--the more it costs. Basic math. You really do get what you pay for--and then some if you choose the right firm to design and build your project.
For homeowners who are considering home additions (a room or porch addition), you really want to align yourself with a company that is able to give you this kind of attention.