I may be able to add some insight. I DIY built a composite 12'x16' deck about 2 years ago (about the same size as what you're proposing). The actual decking was about $3-4/sqft, but the total project cost was about $3000. It's a multitude of other things (besides labor) that drive up the cost. It's not just railings, it's the spindles, the railing posts, the post sleeves, the support posts, the beams, the joists, the joist hangers, the rental of the auger to dig the holes, the sonotubes, the concrete, the anchor brackets, the hurricane ties, the post caps, the concrete for the pad at the bottom of the stairs, the cladding, the plastic we have to put down under the deck, the gravel to put under the deck, the permit, and all the hundreds of nails and thousands of screws (and don't forget driver bits for the screws!). And a ton of labor. On top of that, you can't forget the contractors' non-labor costs as well--the trucks, the time spent ordering and picking up the materials, payroll taxes, time spent putting together quotes, etc.
It wasn't a complex job, but it took a bunch of time, as I was soloing it. The nice thing about DIY is that I was able to make sure it was perfectly flat, and that the posts are all perfectly plumb and rock solid. I spent a bit extra to put my joists 12" on center instead of 16", and to put in extra blocking to make sure the deck stays nice and steady.