Pricing materials simply off sq/ft is misleading. Your contractor might be doing bearers with more support posts, larger posts, higher off the ground than another same sized deck. Is he using 5/4 or 2x decking? That changes the cost a great deal. Stainless deck screws and membrane over the supports? That’s the way I build mine. And of course, regionally material costs differ greatly.
I don’t think $9-$10k is that out of line if it’s a well-built, quality contracted project. And of course the ally railing system is very pricey right now as metals are still peaking in price.
And for reference, I did a 10x10 low deck as a hot tub patio with a pergola last year. Materials were over $4k. But you could drive a dump truck over it and it won’t flex. It will certainly outlast me and my wife.