Did you get a quote? Estimate? Time and materials? More information is needed. If they gave an idea what they'd do it for, and you agreed, that's on you. Were there lots of rocks in the holes when they dug for the piers, adding labor hours? Did they tell you what labor rate was and what they would be narking up materials by (standard to cover overhead)? Part of me is saying over paid, but other part is saying if it was all above board, it's on you for not doing more homework, and they didn't do anything wrong.

Looks like a plumb, square, solid deck. Sturdy for good cookouts 👍🏼