I did the extra soundproofing myself for about a tenth the price the builder wanted...
Just curious, but do you mean that you put the soundproofing in the walls yourself while they were still being constructed?
Exactly. After the house was framed, wired and inspected I put soundproofing inside all the interior walls. IIRC the cheapest solution was R-13 Kraft backed insulation between the studs which worked out to about $100 per thousand square feet of living area.
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"There is not a man of us who does not at times need a helping hand to be stretched out to him, and then shame upon him who will not stretch out the helping hand to his brother." -Theodore Roosevelt