Sarah,
I previously saw some pics you posted of the rail; you have to understand that the glue-ups to make each balluster are very material intensive and time consuming. There is no good way to take solid 4" square stock off the shelf and start turning it. So even before the lathe work commences there is the preparation. Several man-days to get stock. And the cost of perfectly knotfree rot-resistant wood. The rails also use very wide and thick stock. It's extremely space intensive too, waiting for glue to dry.
Then, a wood turner of sufficient skill to duplicate the ballusters demands a pretty high wage. It's not like you can stick just anybody in front of that tool. To get nice turnings is sort of an art form. So $300/ft for custom rail means two things: the shop bid the job to do it not because they need the work, or it is a reasonable market price for the time and materials.
There are plenty of custom architectural wood turners on the web. Specify a tropical hardwood like mahogany or spanish cedar or you'll be redoing it again in 10 years.
Casey

Have you tried vintage woodworks they sell all sorts of victorian trim

Here is a link that might be useful: vintage woodworks

"There is no good way to take solid 4" square stock off the shelf and start turning"

Go to a hardwoods dealer and get thicker material from them.

They often have larger dimension softwoods that will NOT be the typical 4x4 wood from the lumberyard that contains pith and is really only useful as firewood.

Large dimension clear wood is NOT inexpensive.