That seems a little high to me,I know that in my area you can get 125 tank,stand,filters,pumps,etc and sometimes canopy tops for anywhere of $300-$400.
I would offer $375 and then maybe go to $400 tops.
My mom is looking at a 150 gallon,with stand(home made)lights,filters,sand,etc for $125.
Bottom line,it's your choice.