With what you're doing it sounds like it's a big job, probably some people are building new decks that size. The new section needs supports and the stairs and landing add to the cost.

Plus skilled labor and building supplies are expensive these days.

I'm not sure if I can tell you if it's reasonable or not since I don't have expertise in that and I haven't shopped around for something similar, but if you get quotes all around the same price it seems that's the going price. For something like a deck (or anything on your house) I wouldn't necessarily take the cheapest bid of you think someone else who is a bit more money will do the job in a timely fashion and do it well. In my book it's always worth paying a bit more for well done than trying to fix poorly done stuff.