Thank you all.
It seemed very high to me, it was the cost of building an actual addition, but I didn't have any comparison point. I'll take a trip to Leesburg and see the showroom there. My wife is a licensed Florida architect, so she's qualified to draw up whatever we want, but I thought it was usually just a matter of assembling mass produced components on an existing slab under an existing roof.
We'll probably wait a few months for whatever we do since we have an existing acrylic enclosure that works well, but it's probably hot in the summer.
It seems that no one here has used Home Depot so that probably says it all.